The oxidation of nitric oxide and propylene was studied over a model alumina-supported platinum catalyst. Two treatments (22 h at 250 degrees C and 2 h at 800 degrees C) involving sulfur dioxide were performed in order to understand the effect of SO2 aging. The role and stability of sulfur species stored during aging were investigated by the reduction of sulfated samples at 500 degrees C and temperature-programmed reduction (TPR) up to 800 degrees C. The low activity obtained after aging without reduction revealed the poisoning effect of surface sulfur species. The reduction at 500 degrees C released half of the surface species, which increased the catalytic activity for both NO and C3H6 oxidation. The TPR removed the most stable sulfur species and improved the activity during cooling. The catalyst aged in SO2 at high temperature showed the greatest activity for both reactions because of SO2 exposure and low sulfur storage.