The effect of SO2 on catalytic performance was studied for the NO/CH4/O-2 reaction over Co-ZSM-5 and Co-ferrierite with and without the presence of water vapor. Over a Co-ZSM-5 catalyst, in the absence of added H2O the addition of 53 ppm SO2 slightly decreased the NO conversion at 500 degrees C but doubled the NO conversion at 550 and 600 degrees C. With the coexistence of 53 ppm SO2 and 2% H2O, a substantial decrease in NO conversion was found at T less than or equal to 550 degrees C but no effect was observed at 600 degrees C. Over a Co-ferrierite catalyst, the addition of 53 ppm SO2 caused a sharp decrease in NO conversion. With both catalysts, CH4 conversion was significantly reduced, and the CH4 selectivity greatly enhanced as the result of SO2 addition. Temperature-programmed desorption measurements of NO adsorbed on fresh and SO2 exposed catalysts revealed that ca. 26% of Co2+ sites were covered by SO2 in both Co-ZSM-5 and Co-ferrierite catalysts during a steady-state reaction at 550 degrees C. In addition, the degree of impact of SO2 on catalyst performance provides further insight about the catalyst sites.