The catalytic reduction of NOx over 5% Ag/gamma-Al2O3 was studied in the presence of various SO2 concentrations (between 0 and 100 vppm) while using propane/propene as reductant either separately or as a mixture. A promotional effect on the catalytic activity was found for all SO2 concentrations. Propene as reductant was found to perform best. Measurements were performed for 6 h time-on-stream during which maxima in the de-NO., activity were found. While these maxima occurred at different times, they were always at the same level percentage. The results provide evidence for differences in the de-NO, mechanism when replacing propene by propane.