The catalytic performance of Ir/WO3 catalyst has been studied in the reduction of NO with CO in the presence of O-2 and/or SO2. The catalyst was active for the selective reduction of NO to N-2 without foreign gases. The conversion of NO to N-2 was further enhanced by the addition of a small amount of O-2 while that to N2O was reduced. The N2 conversion went through a maximum, which depended on the composition of the reacting gaseous mixture (NO and CO). The catalyst was still active at higher 02 concentrations up to 5%, so this Ir/WO3 catalyst can be used effectively under lean-burn conditions. The presence of SO2 (100 ppm) caused a significant deactivation but this negative effect was completely removed by co-existing O-2. The reaction mechanisms and the effects of co-existing O-2 and SO2 gases are discussed on the bases of those reaction results, those of the reduction of N2O instead of NO, and FTIR measurements of the catalysts after the reactions under various conditions. Possible reaction pathways are proposed, which depend on the concentration of O-2 added in the reaction mixture. (C) 2007 Elsevier B.V. All rights reserved.