In situ IR spectra were recorded during the selective reduction of NO by C3H6 on Al2O3 at 573 K, and strong bands due to the adsorbed acetate were observed, accompanying weak bands due to formate and nitrate. The acetate was also formed in C3H6+O-2, while nitrate was formed in NO+O-2. The reactivity of acetate with NB,species was examined by exposing it to He, NO, O-2 or NO+O-2 while monitoring its rate of disappearance, and the results showed that the acetate reacts preferentially in NO+O-2. A complementary set of experiments by GC showed that N-2, CO and CO2 are produced by the reaction of acetate with NO+O-2. The results demonstrate that acetate and nitrate significantly contribute to the formation of N-2 during the present reaction. A possible reaction mechanism is proposed, which explains the role of O-2 in facilitating the reduction of NO by C3H6.