Chemical Constituents of Essential Oils (EO) Essential oils are mixtures of more than 200 components, normally mixtures of terpenes or phenylpropanic derivatives, in which the chemical and structural differences between compounds are minimal. They can be essentially classified into two groups: Volatile components: Volatile components of EO comprises 90–95% of the oil in weight, mainly monoterpene and sesquiterpene hydrocarbons, as well as their oxygenated derivatives along with alcohols, aliphatic