Ionic liquids, formed by combining 1 mol of 1-butyl-3-methyl-1H-imidazolium chloride, bmim(+) -Cl-, with 2 mol of a Group IIIA metal chloride (Al, Ga, In), were examined for toluene carbonylation reactivity to determine the effect for changing the M(III) cation. These ionic liquids were characterized for Bronsted acidity indirectly by C-13 NMR of labeled acetone, CH3-*CO-CH3. These results were explained by a mechanism where the Bronsted acidity of the ionic liquid was influenced by the strength of the Lewis acid: M3+Cl3. (c) 2007 Elsevier B.V. All rights reserved.