A range of alumina- and silica-supported metal catalysts have been investigated for the selective reduction of NOx by propene in excess oxygen. Platinum, in particular, has been found to have a high activity and selectivity. There is a close correlation between the activity for NOx reduction and propene combustion. For platinum group metals, it is found that silica-supported catalysts are more active than alumina-based materials. For Pt/SiO2 catalysts, it is found that the specific activity for NOx reduction decreases with decreasing metal dispersion. NO is found to inhibit the oxidation of propene on Pt-based catalysts.