In order to clarify the mechanism of the selective catalytic reduction of nitric oxide with ammonia over a V2O5(WO3)/TiO2 commercial SCR catalyst, measurements were made on the reaction rate, r(NO), as a function of partial pressure of nitric oxide, P-NO, partial pressure of ammonia, P-NH3, and partial pressure of oxygen, P-O2, from 513 to 553 K under steady-state conditions. The adsorption of NO and NH3 on the catalyst was also observed by infrared spectroscopy (DRIFT). The apparent reaction orders with respect to NO were observed to be less than unity, 0.6-0.8. The reaction rate was nearly independent on P-NH3 at lower temperatures. As temperature increased, r(NO) became slightly increased with increasing P-NH3 at lower partial pressures of ammonia and tended to be saturated with further increases of P-NH3 The dependence of r(NO) on P-O2 was similar to that of P-NH3: r(NO) increased with increasing P-O2 at lower partial pressures of oxygen and was saturated with further increase of P-O2. The spectroscopic study showed that NO does not adsorb significantly on the oxidized nor on the NH3 preadsorbed surface of catalysts above at least 473 K. The SCR reaction was considered to proceed as follows. NH3 adsorbed on the Bronsted acid sites as ammonium ions. Ammonium ions were activated with the terminal oxygen groups, V5+=O, prior to the reaction with gaseous NO. Subsequent reaction with NO produced N-2, H2O, and the hydroxyl groups bonded to the reduced vanadium, V4+-OH, which would be reoxidized by oxygen to the V5+=O species. The Bronsted acid sites where NH3 adsorbed were then recreated. The results obtained in this study suggested that the Bronsted acid sites and/or the V5+=O species were equilibrated with the other species on the surface, implying that the number of each site changed with the experimental conditions such as P-O2. The relative amount of the V5+=O species would vary from similar to 0.1 to similar to 0.4 with increasing P-O2.