Oxygenates are compounds containing oxygen in a chain of carbon and hydrogen atoms. Today, oxygenates are blended into petrol in two forms: alcohols or ethers.

In alcohols, each oxygen atom is linked to a carbon atom and a hydrogen atom, forming a carbon-oxygen-hydrogen sequence. Ethanol is the most commonly used alcohol. In ethers, each oxygen atom is linked to two carbon atoms, forming a carbon-oxygen-carbon sequence.