The effect of SO2 for the selective reduction of NO by C3H8 on Ag/Al2O3 was investigated in the presence of excess oxygen and water vapor. The NOx conversion decreased permanently even in the presence of a low concentration of SO2 (0.5-10 ppm) at <773 K. The increase in SO2 concentration resulted in a large decrease in NOx conversion at 773 K. However, when the reaction temperature was more than 823 K, the activity of Ag/Al2O3 remained constant even in the presence of 10 ppm of SO2. The sulfate species formed on silver should mainly decrease the deNO(x) activity on the Ag/Al2O3. The sulfated Ag/Al2O3 was appreciably regenerated by thermal treatment in the deNO(x) feed at 873 K. The moderate activity remains at 773 K in the presence of 1 ppm SO2 for long time by the heat treatment at every 20 h intervals.