The mechanism of the selective catalytic reduction (SCR) of NO by C3H6 On Al2O3 was investigated using in situ IR spectroscopy. Attention was focused on the reactivity of the adsorbed acetate and nitrates on the Al2O3 surface. IR spectra showed that the reaction starts with the nitrates formation from NO + O-2 followed by its reaction with C3H6 to form acetate, which becomes the predominant surface species in the steady-state condition. The acetate band, which was stable in He or NO, significantly decreased when the flowing gas was switched to NO + O-2 A complementary set of experiments monitoring gas composition showed that N-2 and COx were produced by the reaction of acetate with NO + Oz. The rate of acetate consumption in NO + O-2 exhibited the same order of magnitude as the NO reduction rate, indicating that the acetate is active as a reductant and takes part in the N-2 formation. Nitrates can oxidize both C3H6 and acetate, and are mostly reduced to N-2. A proposed reaction scheme explains the role of O-2 in facilitating SCR of NO.