Deactivation of copper ion-exchanged mordenite (CuHM) catalysts with different copper contents by SO2 for NO reduction with NH3 was examined in a fixed bed flow reactor. A larger amount of deactivating agent deposits on the catalyst surface with the increase of copper contents under the same operating conditions. The deactivating agents deposited on the copper-exchanged mordenite catalysts were mainly (NH4)(2)SO4 and/or NH4HSO4 from the results of thermal analyses such as thermogravimetric analysis (TGA), differential thermal analysis (DTA), and temperature-programmed desorption (TPD). The formation of the ammonium salts largely depends on the reaction temperature and SO3 concentration generated from SO2 oxidation which is a reaction catalyzed primarily by copper ions on the catalyst surface. The catalytic activity and surface area of the deactivated catalysts are well-correlated with the sulfur content deposited on the catalyst surface, depending upon the reaction temperatures and its catalyst copper contents.