The lean-NOx performance and the effect of SO2 On silver doped gamma alumina catalysts were investigated with simulated diesel exhaust in order to understand the active site and poisoning phenomena of the catalysts. The samples were prepared by impregnating gamma-alumina with silver nitrate or silver sulfate using incipient wetness technique. Among the catalysts tested in the absence of SO2, 2 wt.% Ag/Al2O3 was found to be the optimum silver loading for NOx reduction, while further increasing the Ag loading resulted in lower selectivity due to the fact that metallic Ag species in high loading samples favors propene oxidation. However, the common catalyst performance paradigm dramatically changed when SO2 was present in the feed stream. The NOx reduction over the 2 wt.% Ag/Al2O3 initially was enhanced by SO2 exposure and then decreased until the N-2 Yield leveled off at 40%. Upon removal of SO2 from the feed stream, the catalyst was completely regenerated to the original N-2 yield. The NO, reduction performance of a high silver loading catalyst (8 wt.% Ag/Al2O3) increased significantly after SO2 was introduced in the gas stream from its initial performance of 1-28%. XPS analysis showed that the dispersion of silver species decreased by exposure to the feed gas containing SO2. Crystallite silver sulfate (Ag2SO4) was observed in the XRD pattern for the 8 wt.% Ag/Al2O3. The catalyst performance and characterization results suggested that the silver sulfate species enhanced the NOx reduction capability via producing more intermediate species (-NCO). Eventual decrease in the performance could be attributed to SO2 poisoning of the alumina sites, which are responsible for NOx reduction to N-2. A sample containing selectively sulfated silver sites showed higher NOx reduction performance than a sample containing silver oxide. However, when alumina sites were sulfated it demonstrated lower NOx reduction than a pure alumina. With the results obtained from this study the order of apparent catalytic activity for silver species to enhance NOx reduction on alumina is proposed as the following: Ag2SO4 > Ag2O > Ag-0. Published by Elsevier B.V.