are substances which raise the antiknock quality of a gasoline, as expressed by octane number. Historically, tetraethyl lead (see lead alkyl) has been the most common antiknock compound, but its use is being phased out under Environmental Protection Agency (EPA) regulations. Coming into increasing use as octane boosters are toluene and oxygenated organic type substances such as methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME).

Also see knock.