An infrared study of the liquid complex of aluminum chloride with acetyl chloride has been undertaken. The results indicate that the pure liquid complex is not a simple mixture, but consists of (1) the donor–acceptor complexand (2) the ions [CH3CO]+and [AlCl4]−. A solution of the liquid complex in a solvent of high dielectric constant like nitrobenzene (ϵ = 36.1) contains both these species, but in a low dielectric constant solvent like chloroform (ϵ = 5.05) only (1) is present. The carbon–chlorine bond in (1) is modified. The mechanism of Friedel–Crafts ketone synthesis is briefly examined in the light of these results.