Catalytic activity for NO reduction with propene was investigated at 0-80 ppm SO 2 . NO was reduced more efficiently by propene on SO 2 -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. NH 3 adsorption revealed that SO 2 treatment increased Bronsted acidity of the Ag/Al 2 O 3 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 SO 2 -adsorbed Ag/Al 2 O 3 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.