Long-term isothermal studies of the influence of SO2 on the deNO(x) activity of a medium loaded (5 wt.%) Ag/gamma -Al2O3 catalyst were performed under lean-bum conditions at 480 degreesC, using a propane/propene mixture as reductant. The presence of SO2 in the feed seems to have a promotional effect on the deNO(x) activity. This effect was clear over long-term experiments up to 15 h and the catalyst activity stabilises earlier in the presence of SO2. The formation of hydrocarbon oxygenates, sulphur species accumulation on the catalyst and changes in the deNO(x) reaction mechanism seem to be responsible for the promotional effect. Considerable quantities of sulphur species were accumulated on the catalyst during the first hours time-on-stream, when the promotional effect was more intensive. Interactions between SO2 and NOx and between SO2 and the hydrocarbons were the major sources of sulphur species accumulation, with the SO2/NOx interactions having the larger contribution. TPD studies after catalyst pre-treatment in different reaction mixtures provided evidence for the formation of RSOx species. The interactions between SO2 and the other feed constituents show that the usually applied ex situ sulphation treatment by SO2/O-2 mixtures does not simulate the actual sulphation procedure.