in petroleum refining, chemical process in which light, gaseous hydrocarbons are combined to produce high-octane components of gasoline. The light hydrocarbons consist of olefins (olefin) such as propylene and butylene and isoparaffins such as isobutane. These compounds are fed into a reactor, where, under the influence of a sulfuric-acid or hydrofluoric-acid catalyst, they combine to form a mixture of heavier hydrocarbons. The liquid fraction of this mixture, known as alkylate, consists mainly of isooctane, a compound that lends excellent antiknock characteristics to unleaded gasolines.

Alkylation units were installed in petroleum refineries in the 1930s, but the process became especially important during World War II, when there was a great demand for aviation gasoline. It is now used in combination with fractional distillation, catalytic cracking, and isomerization to increase a refinery's yield of automotive gasoline.