The impregnation and sol-gel preparation methods were investigated to develop high activity catalysts and understand the significance of the indium-aluminium interaction on alumina-supported indium catalysts in NOx reduction with propene. Active In/alumina catalysts with a very high surface area (270 m(2)/g) and thermal stability were prepared in controlled conditions by sol-gel processing. When Al isopropoxide and In nitrate in ethyl glycol were used as precursors in aqua media, indium atoms were incorporated evenly distributed as a thermally stable form in the aluminium oxide lattice structure. In wet impregnation it was beneficial to use a certain excess of aqueous In solution (volumes of solution : pores = 2 : 1) to have the highest NOx reduction activity. The catalyst containing dispersed Al on In oxide (58 wt% In, phase-equilibrium preparation method) showed activity at lower temperatures than any other In-Al oxide catalyst or pure In2O3. The adsorption of different reaction intermediates on alumina and stable In2O3 sites were detected by FTIR studies. In/alumina catalysts have active sites to oxidize NO to NO2, partially oxidize HC, form the actual reductant which contains N-H or N-C bonding and react with NO to dinitrogen. The cooperation with indium and aluminium was evident even in the mechanical mixture of sol-gel prepared alumina (301 m(2)/g) and In2O3 powders (27 m(2)/g), where the probability for molecular-scale intimate contact between indium and aluminium sites was very low (particle size 10-250 mu m). Short-lived gaseous intermediates and surface migration are the possible reasons for the high catalytic activities on the two physically separated active sites both necessary for the reaction sequence.