The alkylation mechanism catalyzed by an ionic liquid (as a Lewis acid) may be different from the traditional alkylation mechanism catalyzed by Brφnsted acid,especially as their initiation steps are still not clear.In this paper,an isotope exchange method is used to investigate the catalytic mechanism of AIC13/butyl-methyl-imidazolium chloride ionic liquid in the alkylation of benzene with 1-dodecene.The proposed catalytic mechanism was confirmed by analysis of ionic liquid before and after reaction and of the alkylation products of deuterated benzene (C6D6) with 1-dodecene.The proposed mechanism consists of the equilibrium reaction between [Al2Cl7]-+H+ and [AlHCl3]++[AlCl4]-,in which the Brφnsted acid [AlHCl3]+ is supplied by the reaction of 2-H on the imidazolium ring and [Al2Cl7].The alkylation reaction is initiated by the Bronsted acid [AlHCl3]+ which reacts with l-dodecene to form a carbonium ion,then the carbonium ion reacts with benzene to form an unstable o complex,leading to the formation of 2-phenyldodecane.