The reaction mechanism and catalytic cycle for the selective catalytic reduction of nitric oxide by ammonia over vanadia/titania catalysts has been elucidated by in situ on-line FTIR studies under steady-state conditions. Under all reaction conditions, a large concentration of ammonia is adsorbed on both Lewis and Bronsted acid sites, whereas no significant amounts of adsorbed NO are adsorbed. The catalytic activity is found to be related to the ammonia adsorbed on the Bronsted acid sites associated with V5+-OH. Surface V=O groups are involved in activation of the adsorbed ammonia and are also found to play an important role in the catalytic cycle. The activation involves a transfer or a partial transfer of a hydrogen and reduced V-OH groups are produced. The V5+=O surface species are regenerated by oxidation. The results, therefore, show that the catalytic cycle consists of both acid-base and redox reactions. The ammonia adsorption is observed to be a fast equilibrated step under all the conditions studied but the other catalytically significant steps may shift depending on the reaction conditions. At high O-2 partial pressures, the rate is mainly determined by the concentration of Bronsted acid sites and the NO partial pressure, whereas at low O-2 partial pressures, surface reoxidation is slow and the rate becomes dependent on the concentration of V5+=O groups.