The effect of the preparation method on the activity of Ga2O3-Al2O3 for the selective reduction of NO with propene was investigated. Ga2O3-Al2O3 prepared by the sol-gel method (Ga2O3-Al2O3(S)) exhibited excellent activity for NO reduction, compared with Al2O3, Ga2O3, and impregnated Ga2O3/Al2O3 (Ga2O3/ Al2O3(I)). XRD measurements of Ga2O3-Al2O3(S) indicated that a part of the Al3+ ions in Al2O3 are substituted by Ga3+ ions, resulting in the formation of a composite oxide, [GaxAl(1-x)](2)O-3 (x < 1). On the other hand, Ga2O3 particles and Al2O3 particles were found to be present separately for Ga2O3/Al2O3(I) The high catalytic activity of Ga2O3-Al2O3(S) was accounted for by the high surface area and the nature of the [GaxAl(1-x)](2)O-3 (x < 1) composite oxide, which was formed uniformly by the present sol-gel method. The comparison of the reactivity between NO and NO2 suggested the participation of NO2 in NO reduction. The kinetic order for NO reduction was found to be 0.3 with respect to propene and 0.7 with respect to oxygen on both Ga2O3/Al2O3(I) and Ga2O3-Al2O3(S). However, a significant difference in the reaction order with respect to NO was observed between Ga2O3/Al2O3(I) and Ga2O3-Al2O3(S), 0.4 order on the former catalyst, and zero order on the latter one, indicating the presence of strongly adsorbed NOx species on the catalyst surface. On the basis of these results, a reaction mechanism was proposed in which the NOx adspecies formed through NO oxidation is reduced to nitrogen via organic intermediates containing nitrogen and oxygen. (C) 2000 Academic Press.