Selective catalytic reduction of NO by methane over CoH-ZSM-5 in the presence of SO2 was studied. SO2-TPSR and NO+ (O2)-TPD techniques were applied to quantify the active sites and probe the poison effect. At 773 K, addition of 78 x 10(-6) SQ2, the NO to N2 conversion ratio decreases from 72% and reaches stable level 58%. Increasing the reaction temperature to 823 K, the NO to N2 conversion ratio increases to 62%. Co-existence of 2.55% H2O and 78 x 10(-6) SO2 at 773 K, the NO to N2 conversion ratio decreases further to 51% , but almost no effect is observed at 873 K. Three SO2 desorption centers at around 690 K, 810 K and 910 K are formed over the SO2-TPSR spectrum of the poisoned catalyst. Even at 970 K, the species contained sulfur is not desorbed completely. The amount of adsorbed NO and active intermediate--NO, species decreases over the poisoned catalyst, indicating that partial active sites are covered by the compound contained sulfur. As a result, the NO conversion ratio decreases. On the other hand, the presence of SO2 inhibits the conversion of CH4 and the compounds contained sulfur is desorbed further at higher temperature, which causes the temperature where the optimum NO conversion shifts to 823 K.