The sulfur tolerance of Ag/Al2O3 catalysts in Hrassisted C3H6-SCR was investigated by UV vis, TPR, TPSR, DRIFTS-MS, and DFT calculations. Ag/Al2O3 with higher silver loadings exhibited better deNOx performance and sulfur tolerance, especially the 4% Ag/Al2O3 catalyst. UV-vis and H-2-TPR revealed that highly dispersed Ag+ cations were predominant on 2% Ag/Al2O3, while more metallic Ag clusters with large sizes were present on the 4% Ag/Al2O3. After exposure to SO2, large amounts of sulfates were adsorbed on the Ag sites and Al sites of the Ag/Al2O3 surface. The sulfates were reduced to H2S and SO2 in a reducing atmosphere, while they showed little decomposition under real SCR reaction conditions. DRIFTS-MS experiments showed that sulfate species transferred rapidly between Ag sites and Al sites on the Ag/Al2O3 catalysts with higher amounts of Ag clusters. DFT calculations revealed that Ag-1 cations show stronger affinity for sulfate species than Ag clusters, thus resulting in blockage by sulfates at the Ag-O-Al interface. Such blocking by sulfates suppressed the activation of C3H6 as well as the formation of -NCO species, and thus severely inhibited the deNOx performance of 2% Ag/Al2O3. In contrast, the rapid mobility of sulfate species on 4% Ag/Al2O3 made more active sites available for the formation of key intermediates of HC-SCR, finally contributing to its excellent sulfur tolerance.