The different impacts of SO2 and SO3 on Cu/zeolite SCR catalysts were investigated by SCR performance tests and multiple characterization techniques including temperature programmed desorption (TPD), Xray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS). The results indicate that a larger amount of highly dispersed CuSO4 formed in the zeolite catalysts (Z-CuSO4) upon SO3 poisoning, explaining the much more significant deactivation of the Cu/zeolite catalysts that were exposed to SO3 compared to poisoning by SO2. This paper provides the first demonstration that active sites of Cu/zeolite SCR catalysts involved in the storage and removal of sulfur can react with SO2 and SO3 in very different ways. In particular, the significant differences in the extent of sulfur uptake account for the considerably different impacts of SO2 and SO3 poisoning on the performance of Cu/zeolite SCR catalysts. (C) 2010 Elsevier B.V. All rights reserved.