A Crash Course In Essential Oil Chemistry

essential oil chemistry

If you want to use essential oils for more than just ‘diffusing for the smell’  You might want to consider learning the basics of essential oil chemistry.

Understanding the component parts of an oil, enables you to predict what kind of emotional or physical effects each oil might potentially have.

You can’t describe the extraordinary sense of pride you feel when your hubby or one of the kids, comes to you with problem, you look in your oils box and you can confidently whip up a blend to treat them.

Even better... it works and you understand why! 

The chemistry of the oil dictates the properties, so learning more about the chemistry side of things also provides you with a better understanding of why some oils might have more safety considerations attached to them than others.  

It could be said that chemistry takes away from the intuitive side of aromatherapy but in fact the opposite is true. By contrast, recognising the chemical patterns enhances most people’s essential oil journeys and helps them to get even more from their oils and to incorporate them more fully into their everyday lives.

This post is designed as a chemistry ‘crash course’ and reference point to welcome you into the world of essential oils and empower you to get the most out of your essential oil use.

It is however, simply a starting point so be sure to read the conclusion at the end which puts this post in context when it comes to the exciting world of aromatherapy.

IMPORTANT NOTE: We are only talking about the general chemical families and their properties in this post. We make no claims as to how to obtain any of the properties mentioned from the chemical families, under what conditions, from what products, or in what doses, or even if the desired properties can be obtained at all in humans. The fact that certain essential oils contain these constituents in no way means that we are claiming that the essential oil product has any specific medicinal or therapeutic properties.


Table Of Contents


It All About The Molecules

Essential oils are made up of molecules but let’s starts small and begin with the question...

“What on Earth is a molecule?”

You can consider these as the smallest component parts that make up the oils.

All living things are made of molecules arranged in many different ways.

For the most part these will be either chains of molecules, or rings, or a combination of the two. They comprise of carbon, hydrogen and oxygen.

We call these the chemical constituents or components (both the words ‘constituents’ and ‘components’ are often used by people to describe the same thing). Similar groups of molecules are categorised together in “families”

Each family has its own traits, properties and benefits. They have certain fragrance profiles - because of how they developed in the plant.

Based on how we understand the family to work in the human body, will also give us clues about any safety concerns we might need to be aware of.


What exactly is an essential oil?

Understanding why the plant originally developed the chemicals helps us to comprehend the properties of the oil further. So let’s start there.

Did you know a plant does not need the essential oil to live?


Weird, huh?

In fact, the more correct terms for the chemicals found in your bottle is Secondary Metabolites.

The plant has strangely developed these as a method of making its life easier once is starts growing. Based on the “Character” or “Personality” of the plant, it will evolve these chemicals, internally to do many different things.

Some, it creates as a means of attracting pollinators, others happen because it is living in a very cold or hot place so it needs to create a temperature control. Some are created to pour into the soil, to prevent any other seeds from germinating. Many are manufactured by the plant as a response to some kind of mould or fungus, to heal tissues that have been munched by insects, or even to repel the critters from doing it again.

Read through that list and consider which you might imagine to be sweet, gentle and welcoming and which might be a bit less palatable.

What’s so exciting, is that scientists hypothesize that we respond to essential oils so readily, not because we are connected to plants… but more that we are similarly evolved to the insects.

And that forms a massive part of the research into essential oils today.

How do bugs respond to the oils and would that be the same for us?

Often the answer is yes.

So, as we consider this new context of the oils, we might imagine how the rose opens her petals to welcome a bee, and encourage it to have a bit of wiggle action on her stamen - her sexual reproductive organs. She needs pollen to be taken from them and rubbed onto another flower so that she can produce rose hips full of seeds. In actual fact that’s the sole function of the flower.

So, as the flower opens, it bursts thousands of aromatic molecules into the air as a chemical messaging system that says “Come on boys...I’m ripe for the picking”

And lo and behold, in they come.

Now, imagine you are opening a bottle of essential oil. Molecules literally burst out of the bottle and evaporate. We call this volatility. Those molecules that drift into the air, are the same as erupted from the plant and we call these aromatic volatiles.

Some molecules are more volatile than others. Light (as in weight) ones drift off like fairy wings and heavier ones, which take longer to evaporate, will be slower to move and therefore take much longer.

If we are blending the oils, these very light molecules tend to make up oils we know as top notes and the base notes are rich in molecules which are much heavier.

You will see how these correlate to the chemical groups below.


One place you will find details of these constituents is in a GC/MS report.

The Gas Chromatography and Mass Spectrometry report tells you what chemical constituents can be found in each oil.

Oil from the same plant species can vary greatly in chemical composition, scent and therapeutic properties based on the country and even region they have been sourced from.

Like your favourite bottle of wine, soil and conditions the plant has been grown in makes a huge difference to the essential oil. For that reason you might find certain countries are favoured for certain oils. This does not necessarily mean they are better or worse as an oil per se, but that they have properties that make them better for a particular condition.

Two oils from the same producer may even vary from season to season. Things like the weather and climate conditions, how it’s grown, when it’s harvested, the distillation and countless other external factors all contribute to the chemical composition of the oil produced.

So, this is why we believe every batch of essential oil produced should have its chemistry analysed. That way the consumer knows exactly what they are buying and the properties that each particular bottle of oil will have.


Understanding essential oil synergy: A beautiful cocktail of different chemicals all interacting together

It can be tempting to imagine an essential oil is only made up of one singular chemical group. In fact, most essential oils contain blends of many different chemical groups.

Most oils will have some of each different chemical family contained within their make-up. Their own plant family often dictates what the predominant chemical family might be.

This beautiful cocktail of different chemicals all interact together. These chemicals also support and moderate each other. This results in an essential oil having many different properties.

We call this interaction synergy.


The Chemical Families

Essential oils are made up of carbon and hydrogen (hydrocarbons). The main ‘building block’ of how the hydrogen and carbon molecules are arranged together is called an isoprene. Many sets of isoprenes, categorised by similarity of how each set is built, are known as Families.

Similar arrangements result in similar properties.



The terpene family is large and contributes hugely to both the fragrance and properties of an oil.

Let’s start with the tiniest and work up. These tiny ones are light in weight and so form the top notes of a blend.



This is the largest chemical family and the constituents from this family are found everywhere in nature. Made up of hydrogen and carbon molecules called hydrocarbons, they contain just 10 carbon atoms (so 2 isoprene chains - an isoprene chain contains 5 carbons).

With such a small molecular structure, they are very light. Hence, if you try to drip just one drop of oil into your blend, you’ll find you probably accidentally pour more. They are so small, they are very hard to control and they come bursting out of the bottle.

Top notes are rich in monoterpenes. Their fragrance dissipates quickly. If you use these in a diffuser, they disappear almost immediately.

A nice example of a monoterpene is d-limonene found in most citrus oils. This constituent gives these oils their lemony, fresh zing.

Qualities of Monoterpenes

  • Lipophilic:
    • Being so tiny, these penetrate the tissues of the body easily. Adding oils rich in monoterpenes into blends improves skin penetration.
  • Deodorising:
    • Monoterpenes are great for diffusing because, in addition to smelling good, they also deodorize the air. Pine oils in particular are spectacular for this.
  • Oxidise quickly:
    • Oils high in monoterpenes, such as citrus oils begin to degrade rapidly. Affected by oxygen and light, they will begin to deteriorate after about six months. When this begins, the oils take on a sharpness and will begin to irritate the skin, and cause skin sensitisation and should not be used topically. Older oils are best used for diffusion and household cleaning.  
  • Shelf life:
    • 6 months - 3 years. Store in a cool, dark place to help prolong their shelf life.
  • Safety:
    • Can be irritating to the skin, especially if older/oxidised
    • Always check the safety recommendations for the individual oil you plan to use

Common Properties of Monoterpenes:

  • Antiseptic
  • Antibacterial
  • Antiviral
  • Anti-inflammatory
  • Decongestant
  • Rubefacient - increase circulation
  • Analgesic
  • Stimulant

Oils high in monoterpenes

  • Bergamot and all citrus oils
  • Black pepper
  • Cypress
  • Many conifers  inc. Balsam & Siberian Firs,  Scotch Pine, White Pine, White Spruce, Black Spruce
  • Juniper Berry
  • Nutmeg
  • Rosemary
  • Tea Tree
  • Thyme

Common examples of monoterpenes:

  • Camphene, Found in camphor
  • δ-3-Carene, found in cedar
  • p-Cymene found in most herbs
  • Limonene - citruses
  • β-Myrcene - Myrtle



Like monoterpenes,these contain only hydrogen and carbon (hydrocarbons) but sesquiterpenes have 5 more carbons (so in total 15 carbons or 3 isoprene chains) 

Now then, with these extra carbons the molecules are bigger and heavier, thus take longer to evaporate and rise from the bottle.

Research shows these are able to cross the blood brain barrier affecting the brain, emotions and hormones, and we use these as very sedative and pacifying oils.

Qualities of Sesquiterpenes

  • Oxidisation:
    • Still susceptible to oxidisation but much lower risk than monoterpenes
  • Shelf life:
    • Some can last as long as 6-8 years shelf life. They evaporate much slower meaning they have a much longer shelf life.
  • Safety:
    • Since sesquiterpenes are gentler and less irritating to the skin, these tend to have fewer safety concerns attached to them
    • Always check the safety recommendations for the individual oil you plan to use


Common Properties of Sesquiterpenes:

Benefits are harder to categorise because they have such a diversity or properties. On the whole though, sesquiterpenes are:

  • Antibacterial
  • Antiseptic
  • Analgesic
  • Anti Inflammatory
  • Cicatrizant

Oils taken from the roots of plants tend to be high in this group and as such they act as if they are anchoring you more. They are mostly:

  • Grounding
  • Sedative
  • Emotion and hormonal balancing


Oils high in sesquiterpenes

  • Balsam Copaiba
  • Ginger
  • Helichrysum
  • Cedarwood
  • Manuka
  • Melissa
  • Vetiver
  • Myrrh: contains beta elemene which suppresses new blood vessels from forming in pregnancy and could suppress fetal growth.
  • Patchouli
  • Rhododendron
  • Ylang Ylang (Complete)
  • Black Pepper - can be irritating to the skin


Some common sesquiterpenes:

  • β-Caryophyllene - black Pepper and copaiba
  • β-Cedrene - Cedarwood
  • D, α-Humulene - Derived from hops
  • α-Zingiberene- Found in ginger

Another sesquiterpene that always causes excitement is azulene. This forms from changes in the chemistry during distillation and turns the oil blue. Azulene is like liquid anaesthetic in that it is soothing, relaxing and calming.

Oils containing azulene are:

  • German Chamomile
  • Wormwood (Considered hazardous because of other constituents)
  • Yarrow 
Terpene Family Number Of Carbons (remember an isoprene unit has 5 carbons so 10 carbons means 2 units) Individual Constituents
Monoterpenes 10 Camphene, δ-3-Carene, p-Cymene, Limonene, β-Myrcene, β-Ocimene, α-Phellandrene, α-Pinene, Sabinene, α-Terpinene, Terpinolene, α-Thujene
Sesquiterpenes 15 Aromadendrene, β-Bisabolene, β-Caryophyllene, β-Cedrene, β-Elemene, β-Himachalene, α-Cadinene, α-Copaene, α-Farnesene, Germacrene D, α-Humulene, α-Zingiberene, y-Muurolene
Diterpenes (rare in essential oils) 20  
Sesterterpenes (in nature but not EOs) 25  
Triterpenes (in nature but not EOs - sometimes present in absolutes) 30  
Tetraterpenes (in nature but not EOs) 40  





These comprise the same 10 carbons as monoterpenes, but also contain a ‘functional group’ known as a hydroxyl. That is: they have additional hydrogen and oxygen molecules added into the mix. Hydroxyls (as they are known) are very reactive, so they “burn out” very quickly.

Qualities of Monoterpenols

  • Oxidisation:
    • Still susceptible to oxidisation but generally a low toxicity group
  • Shelf life:
    • 3-5 years
  • Safety:
    • Generally safe on the skin but menthol can be irritating, so watch out for this and  don’t it use on young children under 5
    • Always check the safety recommendations for the individual oil you plan to use 

General properties of monoterpenols:

  • Anti inflammatory
  • Anti microbial
  • Anti bacterial
  • Antiviral
  • Anti fungal
  • Antiseptic
  • Generally good for skincare
  • Have pleasant, uplifting fragrances
  • Stimulant although some, like linalool can have a sedative effect 

Oils containing high levels of  monoterpenols:



Again, here we have the same 15 carbons as sesquiterpenes but with a hydroxyl added in.

These oils tend to be energetically and emotionally grounding and are very good for energising the lower chakras. The properties of this group are harder to generalise since sesquiterpenols are varied and benefits can be unique to the constituent. In the main though, they are listed below. 

Qualities of Sesquiterpenols

  • Shelf life:
    • 6-8 years
  • Safety:
    • Generally safe on the skin
    • The constituent b-eudesmol (in Blue Cypress) should be avoided with anticoagulant medication, peptic ulcer, bleeding disorders and during pregnancy as it has antiangiogenic properties
    • Always check the safety recommendations for the individual oil you plan to use 

General properties of sesquiterpenols:

  • Grounding
  • Sedative
  • Good for the skin
  • Immune stimulating
  • Anti inflammatory 

Oils containing high levels of  sesquiterpenols:

  • Carrot seed
  • Cedarwood
  • German chamomile
  • Ginger
  • Patchouli
  • Sandalwood
  • Vetiver

2-Phenylethanol is interesting in its own right. This molecule makes up most of the scent of roses but as a chemical, it is mainly water-based. Water molecules do not pass through distillation, so when Rose essential oil is made, this constituent stays in the rosewater. The producer must then put the water through a second distillation to retrieve 2-Phenylethanol. This forms part of the reason for the high cost of rose essential oil.


Monoterpenol Constituents
Geraniol Linalol β-Citronellol δ-Terpineol Terpinen-4-ol Menthol Linalool
Lavandulol Borneol Nerol        
Sesquiterpenol Constituents
α-Bisabolol Zingiberol α-Santalol α-Cedrol β-Eudesmol Farnesol Nerolidol




Aldehydes are fresh and zesty. Their vibrant scent is enigmatically uplifting and soothing at the same time.

Imagine smelling a scented candle after a few months. That sense of the brightness having gone from it, is the rich roundness of the aldehydes having evaporated

Qualities of Aldehydes

  • Shelf life:
    • 6 months to 3 years
  • Safety:
    • This group has a high potential for skin irritations and allergic reactions.
    • Stick to low dilutions (1% or under), carry out a patch test and probably avoid using in the bath even with a carrier.
    • The above is further magnified by the high risk of oxidization with this chemical family.
    • Always check the safety recommendations for the individual oil you plan to use 

General properties of aldehydes

  • Sedative yet uplifting
  • Antifungal
  • Anti Inflammatory
  • Disinfectant 

Oils containing high concentrations of  aldehydes


Aldehyde Constituents
Acetaldehyde Anisaldehyde Benzaldehyde Cinnamaldehyde Citral β-Citronellal
Cuminaldehyde 2-Decanal Dodecanal 2-Dodecanal Geranial Myrtenal
Neral Perillaldehyde Piperonal Salicylaldehyde Vanillin  




Terpenes use combinations of isoprene chains (5 carbon chains) but phenols also have ‘side chains’ which are incomplete sets of 5 carbon units in a chain which makes them more unpredictable.

Like alcohols, these have a hydroxyl group but phenols also contain a ‘benzene ring’ in the molecule.


General safety considerations

This “untidiness” of the chemical structure means that the compound is not as stable, is chemically hot and irritating and with many safety considerations to think about when using phenolic oils.

We use these in very low strength dilutions, use them sparingly, and in small discrete areas to check sensitivity. A full body massage with an irritant oil is... well you get the picture!

Phenol constituent names usually end in ‘ol’ and they evaporate slowly, so have more opportunity to penetrate the tissues.

This can be good or bad because there is a higher likelihood of irritation of the skin and mucous membranes. Consider that the mucous membranes would include the inside of the nose and lungs, the digestive tracts and the genital areas.

Take care inhaling these. Think of the effects of snorting cinnamon!

Allow the vapours to come to you rather than sniffing with gusto.

Check the safety data of each oil and beware of using with someone who has some kind of clotting disorder. Many phenols thin the blood, so when mixed with prescription drugs, this can be dangerous.

Only use phenolic oils, in any capacity, for short periods of time and in very weak dilutions.

The body is not very adept at ridding itself of phenols so they can linger in your system for a while and as such, begin to create a build up. For this reason, prolonged use of this group leads to sensitisation.

Always check maximum dilutions and if you are nervous... double your amount of carrier oil.

Avoid using these oils in the bath and always check the safety recommendations for the individual oil you plan to use 

  • Average shelf life:
    • 3-5 years

 General properties of phenols:

  • Strongly antibacterial
  • Strong antiseptic
  • Strongly antimicrobial
  • Strongly antiviral
  • Antifungal
  • Rubefacient (stimulates blood and oxygen circulation to an area)
  • Stimulant

 Oils containing phenols

  • Clove
  • Thyme CT Thymol
  • Oregano
  • Cinnamon leaf
  • Holy Basil


Phenol Constituents
Carvacrol Chavicol
Cresol Eugenol
Guaiacol Isoeugenol




Esters are created when acids and alcohols combine. They are characterised by a fruity aroma. The acid part of the constituent name usually ends in ‘yl’ and the alcohol part usually ends in ‘ate’.

Some examples of esters developing are as fruit ripens and in lavender, the constituent linalool develops into the ester linalyl acetate over time. Think back to the rose opening up her petals for the bees. You can see how the lavender might grow seductive here too.

Esters are good to turn to when you are looking for deep relaxation, stress relief or before you go to bed.

Qualities of Esters

  • Shelf life:
    • 3-5 years
  • Safety:
    • Tend to be easy going on the skin
    • Methyl salicylate (found in extremely high concentrations in wintergreen and sweet birch) is a common ester which is thought to be more powerful than aspirin for pain killing in laboratory conditions. It does however have some serious safety considerations:
      • Overdoses can cause convulsions and irritation to the intestines and gut
      • Poisoning has a 50% mortality rate with symptoms being nausea, vomiting, pulmonary oedema and pneumonia
      • Thins the blood and can increase the effect of other blood thinning medications so should be avoided by anyone taking these medicines.
      • Avoid topical use with damaged skin
    • Neryl acetate - A highly skin healing constituent but varies enormously in levels dependant on where the plant grew.
    • Always check the safety recommendations for the individual oil you plan to use

 General properties of certain esters:

  • Sedative
  • Antispasmodic
  • Antifungal
  • Anti-microbial

 Oils containing high levels of esters

  • Birch (Sweet)
  • Wintergreen
  • Bergamot
  • Cardamom
  • Roman Chamomile
  • Clary Sage
  • Fir (Siberian)
  • Rose Geranium
  • Jasmine Absolute
  • Lavender
  • Spruce (Black)
  • Spruce (White)
  • Ylang Ylang


Ester Constituents
Benzyl acetate Benzyl benzoate Benzyl cinnamate Benzyl salicylate Bornyl acetate Butyl angelate β-Citronellyl acetate
β-Citronellyl formate Dimethyl anthranilate Eugenyl acetate Geranyl acetate Lavandulyl acetate Linalyl acetate Menthyl acetate
Methyl anthranilate Methyl benzoate Methyl cinnamate Methyl salicylate Neryl acetate cis-Sabinyl acetate α-Terpinyl acetate




Ketones need to be treated with caution because they can be quite harsh on the body. The Federation of Aromatherapy has a number of oils listed as hazardous and many of these are due to the high ketone content.


Qualities of Ketones

  • Shelf life:
    • 3-5 years
  • Safety - hard to generalise so care needs to be taken with ketones on an individual basis but some guidelines are below:
    • Pervasive, highly penetrating and very harsh on the body
    • Diabetics should avoid high levels of ketones as when insulin is interrupted, ketone levels can be extremely high
    • Rosemary, camphor and hyssop are three examples of oils containing ketones that are neurotoxic to the CNS. These should be avoided by people with epilepsy and other mental health conditions.
    • Pugelone found in Penny Royal-  can cause heavy bleeding so can be problematic for use by women. Pugelone can also be toxic to the liver and kidneys
    • Thujone also promotes heavy bleeding in women

 General properties of certain ketones - in low concentrations:

  • Good for healing scar tissue
  • Expectorant and mucolytic
  • Cooling

 Oils containing ketones

  • Peppermint
  • Rosemary
  • Rock Rose
  • Sage (Spanish)
  • Spearmint
  • Spike Lavender
  • Turmeric
  • Valerian (Root)
  • Vetiver

 Some common examples of ketones you might see listed in oils are:

  • Carvone - spearmint
  • Fenchone - Sweet Fennel
  • Verbenone - many chemotypes of rosemary


Ketone Constituents
Artemisia ketone Camphor Carvone Fenchone α-Ionone
cis-α-Irone cis-Jasmone Menthone 6-Methylhept-5-en-2-one 2-Nonanone
Perilla ketone Pinocamphone Pinocarvone Piperitone cis-Pulegone
β-Thujone Thymoquinone ar-Turmerone 2-Undecanone Verbenone



Lactones & coumarins

A lactone is a carbon ring with an ester attached and coumarins are a type of lactone.

Lactones and coumarins act in a similar way to ketones and can be neurotoxic, irritating and sensitizing. However the risk is very low, as most essential oils have extremely low levels of these components due to the weight of the molecules being too heavy for them to pass to the oil via steam distillation. Some absolutes do contain higher levels of lactones though.

Lactones are great mucolytics and are useful for easing congestion.

Coumarins (furocoumarins specifically) have sedative properties.

Furocoumarins can cause phototoxicity and skin sensitization. You’ll often see essential oils marked as FCF (furocoumarin free) where these constituents have been removed to make the oil safer to use.

Furocoumarin Constituents
Angelicin Bergamottin Bergapten Bergaptol Byakangelicin
Imperatorin Isopimpinellin Methoxsalen Oxypeucedanin Psoralen



Ethers & Oxides

Ethers occur where an oxygen atom binds to 2 carbons in the molecule.

Oxides are ethers where the oxygen atom forms part of a ring in cyclic form. The most well known in aromatherapy tending to be 1,8-cineole (eucalyptol)


General properties of certain ethers:

  • Respiratory stimulants
  • Decongestant and mucolytic
  • Mentally stimulating


Oils containing ethers

  • Basil (Sweet)
  • Nutmeg


Common Ethers

  • Anethole - found in dill
  • Estragole - Sweet basil
  • Methyleugenol - Holy Basil
  • Myristicin - Nutmeg


Oils containing high levels of oxides


Safety considerations - see individual profiles but some notable items are:

  • Asthma sufferers should avoid 1,8 cineole (common in tea tree and eucalyptus for example) as it can trigger an attack
  • Can be irritating to the skin and sensitizing if oxidized
  • 1,8-cineole rich oils should be avoided around young children
  • Large doses of ethers can be neurotoxic and cause convulsions and death
  • Myristicin and elemicin found in nutmeg oil can be psychotropic if ingested in sufficient quantities although topically applied should not produce these effects
  • Safrole is a liver carcinogen
  • Shelf life
    • 2-5 years but check the individual oil


Example Ethers   Example Oxides
Anethole Dillapiole   α-Bisabolol oxide A α-Bisabolone oxide A
β-Asarone p-Cresyl methyl ether   β-Caryophyllene oxide 1,8-Cineole
Elemicin Estragole   Geranyl oxide Linalool oxide A
Methyleugenol Methyl isoeugenol   Nerol oxide cis-Rose oxide
Phenylethyl methyl ether Myristicin   Sclareol oxide  




This is just a general crash course in essential oil chemistry to help you better understand and use essential oils in your day to day life. Remember though that the generalisations in this post, despite being useful and sure to help you, are only scratching the surface of aromatherapy and essential oil chemistry.

Even the trace constituents in an oil can dramatically contribute to the pharmacological activity and the dominant constituents don’t always cause the strongest effects. One example is orange oil which is around 95% d-limonene, but it is the 5% of other constituents that work together to create the main fragrance.

Lots of individual constituents within each family also come with individual properties or may react in different ways when blended together and biologically (different reactions can be seen depending on how the oil is used and in different individuals)… and this is before we even consider emotional, mental and other factors in a more holistic view.

New research is being produced all the time so by all means use this as a starting point, but know that generalisations are not the most accurate or ideal way to approach things, so always try and look up the individual constituents, never stop learning and always be experimenting on your wonderful and exciting essential oil journey.


Finally just to reinforce this statement:

We are only talking about the general chemical families and their properties in this post. We make no claims as to how to obtain any of the properties mentioned from the chemical families, under what conditions, from what products, or in what doses, or even if the desired properties can be obtained at all in humans. The fact that certain essential oils contain these constituents in no way means that we are claiming that the essential oil product has any specific medicinal or therapeutic properties.




Essential Oil Chemistry Quick Glossary

  • Adulteration:
    • Where substances (which could be synthetic constituents or other plant derived constituents) are added to the oil, usually with the objective of increasing volume and therefore profits
  • Volatile:
    • A substance that evaporates. A higher volatility means it evaporates quicker.
  • Aromatic:
    • A class of hydrocarbons that usually contain a benzene ring. The term ‘aromatic’ originates from the first benzene derivatives isolated from plants as initially they were found to have a pleasant aroma. Over time other derivatives were found that didn’t have a pleasant aroma but the terms stuck.
  • Organic:
    • In chemistry the term ‘organic’ refers to any chemical compound that contains carbon
  • Molecule, Chemical Component, Chemical Constituent:
    • A particle in a chemical compound. Molecules are made up of atoms that are held together by chemical bonds. These terms are often used interchangeably to describe the same thing
  • Phytols (not to be confused with Phytol) and CO2 extracts:
    • A relatively new method of extraction that more closely matches the chemical makeup of the plant compared to essential oils (lots of constituents are found in the plant that are not found in essential oils due to being lost in extraction). Little data exists on these substances at present but they are worth putting on your radar.


What is a chemotype?

A chemotype is the term used to describe different plants of the same species (that produce different chemical constituent profiles).

Oil from the same plant species can vary greatly in chemical composition, based on where it grew. Like wine, the quality of a essential oil relies on the terroir, the climate and soil where the plant grew.

Often, essential oils from the same plant will be radically different, based on where they grew. For this reason, you will see Latin names written fully:

E.g. Sweet Basil - Ocimum basilicum CT linalool

This denotes this particular strain of basil has been analysed and confirmed to be high in the chemical constituent linalool.

In the 1990s, when drugs companies first became interested in the properties of plants, they dispatched thousands of plant hunters to far reached places of the earth. Their jobs were to learn the tricks of the healing trades from tribal medicine men.

What proved challenging was learning why the medicine man would want the version of the flower which had red petals and grew in the shade, as opposed to any other plant of the same species.

We now understand how these climactic rigours change and effect the chemistry, and thus the properties of each plant.

You may be aware of lavender and all the different types of lavender, all with slightly different colours and flowers and each one producing a different essential oil. Well each ‘type’ of lavender is a chemotype.

Every time a new chemotype of a plant species is developed (whether intentionally by man or simply by nature and the plant adapting to the environment), a new essential oil with different properties is also formed.

Important to note is that some chemotypes of the same species can be relatively safe to use and another chemotype can carry considerable safety considerations. Example Spike lavender (Lavandula latifolia) has high levels of ketones whereas Lavandula angustifolia is generally safe for common use.




  • A Complete Guide To Aromatherapy - Salvatore Battaglia
  • Essential Oil Safety - Tisserand & Young (second edition)
  • The Complete Guide To Clinical Aromatherapy & Essential Oils For The Physical Body - Elizabeth Ashley
  • Aromatherapy For Health Professionals - Shirley & Len Price (fourth edition)
  • Advanced Aromatherapy - Kurt Schnaubelt


Further Reading

  • Bowles. J - The basic chemistry of aromatherapeutic essential oils
  • Clarke. S - Essential chemistry for safe aromatherapy
  • Williams. D - The chemistry of essential oil