Carboxylation of benzenes with CO was successfully achieved by a combined catalytic system of Pd(OAc)(2) with H5PMo10V2O40-nH(2)O (HPMo10V2) under the influence of O-2 to form the corresponding benzoic acids in relatively high selectivity and moderate yields. The regioselectivity in the carboxylation of alkylbenzenes with CO was dominated by the steric factor rather than the electronic effect of substituents. However, the carboxylation of benzoic acid proceeded with high ortho-selectivity to give phthalic acid in preference to terephthalic acid, which shows coordination of Pd species to the carboxyl group of benzoic acid. (C) 2007 Elsevier B.V. All rights reserved.