The reaction mechanism of catalytic reduction of nitrogen oxides (NO or NO2) by propene in the presence of O-2 has been studied by means of in situ diffuse reflectance IR, combined with transient response analysis. Three surface species, organic nitro (I) (1565 cm(-1)), carbonyl (II) (1740 cm(-1)), and isocyanate species (III) (2174 cm(-1))were the main species detected during the reaction. Organic nitro (I) and isocyanate species (In) were highly reactive toward NO2 and O-2 to form N-2 and N2O, suggesting that these are the intermediates.