The oxidation of o-xylene and of o-tolualdehyde over V2O5/ TiO2 (anatase) catalysts has been studied at 533-633 K under steady state conditions; catalysts were prepared either by wet impregnation employing NH4VO3 solution or by a grafting technique with VO(O-i-C4H9)(3). At lower temperatures and lower contact times, for both o-xylene and o-tolualdehyde, part of the reactant feed disappeared due to the formation of an involatile polymeric residue, the formation of which was enhanced at lower temperatures, lower contact times, and higher organic reactant concentrations, At lower contact times, the part of the residue deposited on the catalyst surface constituted an important CO2 source, The catalyst prepared by grafting showed higher phthalic anhydride selectivity and less of the residue, The formation of nonselective oxidation products seems to be favoured by uncoated TiO2 since 0.6% V2O5/TiO2 showed higher CO2 selectivity than other catalysts. Crystalline V2O5 caused greater residue formation.