The hydrocarboxylation of styrene has been carried out in the presence of PdCl2(PPh3)(2) as catalyst and the heteropolyacids of general formula H3+nPMo12-nVnO40 (HPA-n, n = 1-4) as co-catalyst in THF as solvent. The heteropolyacid H5PMO10V2O40 and HCl enhanced the catalytic reaction at 80 degreesC leading to excellent selectivity of branched aldehyde (>99%). The presence of HCl is essential for the achievement of the total conversion. No additional PPh3 is needed with PdCl2(PPh3)(2) as catalyst. (C) 2002 Elsevier Science B.V All rights reserved.