Ir/WO3 catalyst is highly active for the reduction of NO by CO even in the presence of either O-2 or O-2 + SO2. However, the activity of Ir/WO3 is fairly lowered by the presence of SO2 alone. It is believed that the active sites lose their activity by the adsorption of SO2 but O-2 promotes the desorption of SO2 from these sites as suggested by TPD, thus the negative effect of SO2 being suppressed by the coexistence of O-2.