The activities of 2 and 6 wt% Ag/Al2O3 catalysts for lean NO reduction with C3H6 were compared, High conversions of NO to N-2 were obtained over 2 wt% Ag/Al2O3. In contrast, the NO conversions to N-2 were much lower over 6 wt% Ag/Al2O3, and this catalyst formed a substantial amount of N2O. The difference in the behavior of the two catalysts was attributed to the much higher Ag dispersion for the 2 wt% than the 6 wt% sample, such that the oxidation states of Ag were different under reaction conditions. The 2 wt% Ag/Al2O3 was believed to contain silver in the +1 oxidation state under reaction conditions, while the 6 wt% Ag/Al2O3 catalyst contained Ag-0 particles, the amount of which decreased at higher temperatures. The presence of Ag-0 resulted in a high rate of C3H6 combustion at the expense of NOx reduction. A synergistic effect was observed over a mixture of Al2O3 and 2 or 6 wt% Ag/Al2O3 and was attributed to the transfer of a very short-lived intermediate from Al2O3 to Ag/Al2O3 or vice versa.