Acylations of aromatic substrates with ketenes involve the reactivity of species similar to vinyl cations. Resonance stabilization of ketene – aluminum chloride complexes seems to make these complexes less reactive than corresponding vinyl cations.The kinetic isotope effect of the reaction with dimethylketene and benzene is 1.06, compatible with vinylcation cases, but not with acylation with CH3COBF4types of electrophiles.Substrate specificity was determined fromk(toluene)/k(benzene) values. It was 47.2 for dimethylketene and 173.7 for diphenylketene. The diphenylketene – aluminum chloride complex could be isolated.