Transient experiments performed over synthesized and commercial V2O5-WO3/TiO2 catalysts during catalyst conditioning and during step changes of the operating variables (SO2 inlet concentration and temperature) show that conditioning of the catalyst is required to attain significant and reproducible steady-state data in both the reduction of NOx and the oxidation of SO2. The response time of conditioning for NOx reduction is of a few hours and that for SO2 oxidation is of several hours. Fourier transform infrared spectroscopy temperature programmed decomposition, and thermogravimetric measurements showed that catalyst conditioning is associated with a slow process of buildup of sulfates : the different characteristic conditioning times observed in the reduction of NOx and in the oxidation of SO2 suggest that the buildup of sulfates occurs first at the vanadyl sites and later on at the exposed titania surface. Formation of sulfates at or near the vanadyl sites increases the reactivity in the de-NOx reaction, possibly due to the increase in the Bronsted and Lewis acidity of the catalyst, whereas the titania surface acts as SO3 acceptor and affects the outlet SO3 concentration during catalyst conditioning for the SO2 oxidation reaction. The response time to step changes in SO2 concentration and temperature is of a few hours in the case of SO2 oxidation and much shorter in the case of NOx reduction. The different time responses associated with conditioning and with step changes in the settings of the operating variables have been rationalized in terms of the different extent of perturbation of the sulfate coverage experienced by the catalyst.