Alkylation is the transfer of an alkyl group from one molecule to another.

In a standard oil refinery process, alkylation combines low-molecular-weight olefins (primarily a mixture of propylene and butylene) with isobutane in the presence of a catalyst, either sulfuric acid or hydrofluoric acid. The product is called alkylate and is composed of a mixture of high-octane, branched-chain paraffinic hydrocarbons. Alkylate is a premium gasoline blending stock because it has exceptional antiknock properties and is clean burning. The octane number of the alkylate depends mainly upon the kind of olefins used and upon operating conditions. For example, isooctane results from combining butylene with isobutane and has an octane rating of 100 by definition. There are other products in the alkylate, so the octane rating will vary accordingly.

Most crude oils contain only 10 to 40 percent of their hydrocarbon constituents in the gasoline range, so refineries use cracking processes, which convert high molecular weight hydrocarbons into smaller and more volatile compounds. Polymerization converts small gaseous olefins into liquid gasoline-size hydrocarbons. Alkylation processes transform small olefin and iso-paraffin molecules into larger iso-paraffins with a high octane number.

Combining cracking, polymerization, and alkylation can result in a gasoline yield representing 70 percent of the starting crude oil. More advanced processes, such as cyclization of paraffins and dehydrogenation of naphthenes to form aromatic hydrocarbons in a catalytic reformer, have also been developed to increase the octane rating of gasoline. Modern refinery operation can be shifted to produce almost any fuel type with specified performance criteria from a single crude feedstock.

In the whole range of Refinery processes Alkylation is a very important process to enhance the yield by regrouping of molecules as desired.