The activity and stability of Ce-Ag-ZSM-5 catalysts for the selective catalysts reduction (SCR) of NO with CH4 in the presence of water vapor and SO2 was studied in this work. Pre-steaming or wet aging of the catalyst in the reaction gas mixture caused a drop in activity. The loss of catalytic activity was partially irreversible and with an increase of water content and/or temperature the irreversible loss became predominant. However, at all tested conditions, including at a temperature of 600 degrees C and in the presence of 8% water, the catalyst was capable of retaining a stable NO conversion to N-2 during a 100 h-long test. In the presence of SO2. the activity of Ce-Ag-ZSM-5 decreased significantly at 500 degrees C, but it was recovered by increasing the temperature to above 550 degrees C. NO uptake measurements and characterization of steamed, sulfated and reaction-aged catalyst samples by XRD, HRTEM, STEM/EDS and XPS were carried out to study the deactivation. The NO uptake capacity of pre-steamed samples decreased as steaming temperature was raised from 400 degrees C to 600 degrees C, but that of pre-sulfated catalysts increased with temperature reaching 80% of the fresh material-uptake capacity after the 600 degrees C-sulfation. Water induced sintering of silver. This, in turn, caused the lower NO uptake and the irreversible loss of the SCR activity observed after severe steaming. In the case of SO2, inhibition of the SCR reaction at low temperatures is caused by SO2 adsorption on active sites. However, this is fully reversible with temperature. No permanent loss of activity due to structural modification took place in SO2-containing reaction gases.