Selective reduction of NO with propylene in the presence of oxygen was carried out over Co ion-exchanged HY zeolites (Si/Al=2.55, 20) and Co-Pt combination catalysts based on HY zeolite (Si/Al=2.55). Between the two Co-HY catalysts, selective reduction of NO to N2 was favored over Co-HY zeolite with lower Si/Al ratio, however, oxidation of NO and combustion of C3H6 proceeded easily over that with higher Si/Al ratio. It was further shown that most of Co existed as ion-exchanged Co [Co2+ or Co2+(OH)¿] over the catalyst with lower Si/Al ratio, and as Co oxide [Co3O4] over the catalyst with higher Si/Al ratio. When bimetallic Co-Pt/HY catalysts were applied, Pt was present in metallic form in the working catalysts. Over these catalysts, conversion of NO increased with increase in the amount of Pt loading, however, selectivity to N2 decreased. The maximum yield of N2 was achieved over 0.1 wt.% Pt¿1.0 wt.% Co-HY catalyst. It is concluded that metallic form of Pt and ion-exchanged Co are essential to obtain high yield of N2, and that the role of Pt is to catalyze the oxidation of NO to NO2, which is successfully reduced by propylene over Co sites to nitrogen.