Which chemical compounds make up most essential oils?

Terpenes are by far the most dominant chemical constituents found in essential oils. These organic compounds are built from multiples of isoprene units, which are simple molecules containing five carbon atoms.12

While terpenes are the primary components, essential oils are complex mixtures that may also contain a variety of other substances, including:

• Hydrocarbons and oxygenated compounds: These include alcohols, aldehydes, ketones, acids, esters, oxides, lactones, acetals, and phenols, which are largely responsible for the characteristic aromas and flavors of the oils.1
• Other chemical groups: Depending on the specific plant, oils may contain straight-chain, aromatic, or heterocyclic compounds. For example, allyl sulfides are characteristic of garlic oil, while phenols are common in many other varieties.1

Most essential oils consist of a mixture ranging from a few dozen to several hundred individual compounds. In some cases, one or a few components may predominate—such as d-limonene in orange oil or methyl salicylate in oil of wintergreen—but trace ingredients are essential for providing the oil's natural and characteristic odor.1