The promotion of sulfur oxides on the selective catalytic reduction (SCR) of NO by hydrocarbons in the presence of a low concentration of sulfur oxides over Ag/Al2O3 has been investigated by a flow reaction test and in situ infrared spectroscopy. When the C3H6 (or C10H22) + NO + O-2 feed-flow reaction was tested, maximum NO reduction was below 30% over fresh Ag/Al2O3. After the addition of SO2 to the feed flow, conversion increased slightly. Conversion increased further after SO2 was cut-off from the feed flow. This demonstrated that the increase in NO reduction activity of the catalyst was related to SOx adsorbed on the catalyst. SOx adsorbed on the catalytic surface (1375 cm(-1)) was detected by IR spectroscopy and was stable within the temperature range. NCO species, as an intermediate in NO reduction, on SOx-adsorbed Ag/Al2O3 in a C3H6 + NO + O-2 feed flow was observed in in situ IR spectra during the elevation of the reaction temperature from 473 to 673 K, while it was only observed at 673 K on fresh Ag/Al2O3 under the same experimental conditions. We suggest that SO, in low concentrations depressed the combustion of reductants by contaminating hydrocarbon combustion active sites on the catalyst, resulting in an increase in NO reduction efficiency of the reductants. (c) 2006 Elsevier B.X. All rights reserved.