A systematic investigation of the selective catalytic reduction (SCR) of NO by NH3 over V2O5/TiO2 catalysts of variable vanadia loading has been carried out at 623 K. Kinetic studies were conducted both in the presence and in the absence of H2O and SO2, The structure of the various catalysts, as well as adsorbed species present on their surface, was characterized by in situ Raman and infrared spectroscopies, Under dry and SO2-free conditions, the turnover frequency of the SCR reaction was found to go through a maximum with vanadia surface coverage at approximately half a monolayer, The observed decrease in the SCR turnover frequency at vanadia surface coverages exceeding half a monolayer can be attributed to the loss of strong acid sites which are associated with the TiO2 support. Addition of H2O to the reacting gas mixture results in a decrease in the SCR turnover frequency of approximately 40-50%, which is independent of the vanadia surface coverage. In situ Raman results suggest that such a decrease can be attributed to the competitive adsorption of H2O on the active vanadia sites, The presence of SO2 in the gas phase during the SCR reaction results in a significant increase of the turnover frequency at low vanadia surface coverages, while it has no effect at vanadia surface coverages above half a monolayer, Raman and infrared results suggest that the effect of SO2 can be attributed to the formation of surface sulfate species, which are only present on the titania surface below half a monolayer coverage due to repulsive interactions between the surface vanadia and sulfate species.