Selective reduction of NO (0.1%) with propylene (0.1%) in the presence of oxygen (5%) with He balance was carried out over Ag supported on Al2O3, H-ZSM5 and H-Y catalysts. Although their activities were different, all three catalysts showed high selectivity to N2 (around 95%). Through several characterizations, it was concluded that silver was present in the form of Ag2O clusters, but not in metallic form for the three catalysts. High selectivity to N2 on all the catalysts is attributed to the same active phase of silver oxide. The difference in catalytic activity is considered to be due to the amounts and the size of Ag2O clusters. Normalized activity per Ag site (TOF) was not the same (Ag/Al2O3>Ag-ZSM5>Ag-HY), which is explained by the carbon depositions and facile transformation of Ag species under redox condition over zeolite-based catalysts.