SO2 shows a significant promoting effect on the activity of V2O5/AC catalyst for NO reduction with ammonia at low temperatures (180-250 degreesC). In the present study, the mechanism of the SO2 promotion was studied, It was found that the promoting effect of SO2 on the catalytic activity is due to the formation of a sulfate species on the catalyst surface. The sulfate species is linked to carbon surfaces other than vanadium or mineral surfaces. There is a synergetic role between carbon and V2O5 for the formation of surface sulfate species. A possible mechanism is proposed. SO2 is adsorbed and oxidized by oxygen to SO3 on the vanadium surface, and the formed SO3 shifts to the carbon surface and converts into sulfate species. The formed sulfate species acts as a new acid site, improves significantly the NH3 adsorption, and hence promotes the activity of the catalyst. During the reaction in the presence of SO2 at low temperatures, the sulfate species stays on the catalyst surface, while the ammonium ions react with NO continuously to avoid the formation and deposition of excess ammonium sulfate salts on the catalyst surface. (C) 2001 Academic Press.