The transient phase of the oxidation of o-xylene and o-tolualdehyde, before establishment of the steady state, has been studied over V2O5/TiO2 (anatase), At lower temperatures, an initial decrease in conversion and phthalic anhydride selectivity were observed, accompanied by an increase in selectivity to involatile products; at higher temperatures, the steady state was rapidly attained and no strong variations of conversion and selectivity were detected, For o-tolualdehyde oxidation, lower selectivities to involatile products were obtained, indicating that the adsorbed o-xylene molecules can lead more effectively to the formation of heavy organic compounds. Results obtained for selectivity to involatiles, time to reach steady state, and conversion indicate that catalyst deactivation can be ascribed to the formation of an organic residue on the catalyst surface, Such a residue can also desorb or undergo oxidation to CO2.