The effect of SO 2 on the V 2 O 5 /AC catalyst for the NO reduction with NH 3 at low temperatures (150–250°C) is studied using a transient method. The results show that the presence of SO 2 in the reaction stream can significantly promote catalytic activity, which is largely conditioned by V 2 O 5 loading and reaction temperature. Lower V 2 O 5 loading and higher reaction temperature are favorable. Higher V 2 O 5 loading causes heavy deactivation of the catalyst. The reason for the behavior of the catalyst is also studied and discussed. The promoting role of SO 2 seems to result from the sulfate species formed and deposited on the carbon surface, which increase the surface acidity and hence the activity of the catalyst.