Using an anodic alumina supported silver catalyst with a low Ag loading (1.68 wt%), NO, adsorption and NOx-TPD/-TPSR measurements in different gas streams were conducted to investigate the formation, consumption and reactivity of surface adsorbed NOx species. And, the reactivity difference between NO and NO2 was also discussed by using non-steady and steady state tests. No remarkable uptake of NO was found in "NO only" adsorption. Introducing oxygen greatly promoted the formation of surface nitrite and nitrate species. Much more surface nitrate species were observed in NO2 adsorption, accompanied by gaseous NO released. The oxidation reaction of NO with oxygen into NO2 was believed to play an important role in the formation of nitrate during the coadsorption of NO and O-2. The result of TPSR indicated that the surface nitrate species could be effectively and preferentially reduced by propene. When introducing oxygen into propene stream of TPSR test, the significantly increased amount of reacted nitrate undoubtedly showed the importance of oxygen in activating propene. The selective reduction of NO, under oxygen-rich conditions was considered to pass through the selective reduction of the adsorbed nitrate species with the activated propene with oxygen. The reactivity difference between NO and NO2 was attributed to the stronger NOx adsorption capacity and oxidation ability of NO2 than NO. With oxygen increasing, however, the difference gradually decreased, and finally disappeared in extremely excess oxygen. Besides the decreased difference in NOx adsorption capacity and gaseous reactivity between NO2 and NO, a slower activation rate of propene with oxygen was a supplementary interpretation for the complete disappearance of the difference in 15% oxygen.