The effect of SO2 on the V2O5/AC catalyst for the NO reduction with NH3 at low temperatures (150-250 degrees C) is studied using a transient method. The results show that the presence of SO2 in the reaction stream can significantly promote catalytic activity, which is largely conditioned by V2O5 loading and reaction temperature. Lower V2O5 loading and higher reaction temperature are favorable. Higher V2O5 loading causes heavy deactivation of the catalyst. The reason for the behavior of the catalyst is also studied and discussed. The promoting role of SO2 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.