Catalytic reduction of NO by propene in the presence of oxygen was studied over SnO2-doped Ga2O3-Al2O3 Prepared by sol-gel method. Although SnO2-doped Ga2O3-Al2O3 gave lower NO conversion than Ga2O3-Al2O3 in the absence of H2O, the activity was enhanced considerably by the presence of H2O and much higher than that of Ga2O3-Al2O3 The presence of SnO2 and Ga2O3-Al2O3 species having intimate Ga-O-Al bondings was found to be essential for the promotional effect of H2O. The promotional effect of H2O was interpreted by the following two reasons. The first one is the removal of carbonaceous materials deposited on the catalyst surface by H2O. The other is the selective inhibition by H2O of the reaction steps resulting in propene oxidation to COx without reducing NO.