The selective reduction of NO with propene has been investigated on Al2O3 and Ga2O3-Al2O3, using in situ diffuse reflectance Fourier transform infrared (FT-IR) spectroscopy combined with on-line mass spectrometry and IR gas analysis. During the NO-C3H6-O-2 reaction, the main surface species detectable by IR were adsorbed nitrate, acetate, formate, cyanide (-CN), and isocyanate (-NCO). Ga2O3 was found to promote the formation of these surface species, especially nitrate, -CN, and -NCO. From the experiments focusing on the reactivity of surface NOx(ads) (nitrite and nitrate) species, we found a very high reactivity of those compounds toward C3H6, leading to the rapid formation of -CN and -NCO species. On the other hand, acetate and formate species were deduced to be spectator species in this reaction. An appearance of upsilon(NH) bands ascribed to the formation of the surface complexes including amido groups (-NH complexes) was observed simultaneously with a decrease of the -NCO band, suggesting that the -NCO species is hydrolyzed to the -NH complexes by the reaction with some traces of water. A reaction mechanism has been proposed: NOx(ads) species formed by NO-O-2 reaction on the catalyst surface react with C3H6-derived species to generate the -NCO species via acrylic species and organic nitro compounds, and then the surface -NH complexes generated by hydrolysis of the -NCO species react with NOx(ads) species or NO2 to produce N-2.