The deactivation of mordenite-type zeolite catalysts for the selective reduction of NO by hydrocarbons in the presence of SO2 was examined in a packed-bed flow reactor system. The physicochemical properties of the deactivated catalysts by SO2 were extensively characterized by TGA, TPSR, XPS, Raman,XANES, the measurements of surface area and elemental analysis. Not only the surface area and sulfur content of the deactivated catalysts, but their TGA and TPSR patterns strongly suggest the formation of a sulfur species as a deactivating agent on the catalyst surface. It is also observed that the sulfur species exists in the form of sulfate (SO42-) by XPS and Raman. It mainly causes the loss of NO removal activity of the catalysts. The sulfate species formed on the deactivated catalysts by SO2 did not significantly alter the chemical environment of the copper ions contained in the zeolite catalysts such as CuHM and CuNZA. It does not exist in the form. of cupric sulfate pentahydrate on the catalyst surface as revealed by Cu K-edge absorption spectra Of the catalysts.