Catalytic activity for NO reduction with propene was investigated at 0-80 ppm SO2. NO was reduced more efficiently by propene on SO2-treated than untreated catalyst. Simultaneously, combustion of reductant was observed to lower NO reduction efficiency. Thus, the role of surface-adsorbed SO (x) species was regarded as depressing reductant combustion. NH3 adsorption revealed that SO2 treatment increased Bronsted acidity of the Ag/Al2O3 catalyst, which promoted propene activation. Reductant activation is a more important step, compared with NO activation to oxidative nitrate species. The NCO species, an index intermediate in NO (x) reduction, was produced on SO2-adsorbed Ag/Al2O3 at a lower temperature (473 K) than on the untreated catalyst. The reductive intermediates at low temperature are suggested to be alcohol, or aldehyde-adsorbed species, based on observed C=O band.