The role of surface species in the selective reduction of nitrogen oxides by propene in the presence of excess oxygen over Pt/SiO2 has been studied at 393 K mainly with IR spectroscopy. Organic nitro, nitrite and carbonyl species were detected during the reaction. The reactions of those three species with nitrogen dioxide and oxygen took place rapidly, producing N2, N2O and CO2, while the reactivities of those species with nitric oxide and propene were low. Hence, a mechanism is proposed, in which the nitro, nitrite and carbonyl surface species are key reaction intermediates. Similarity in the products between the selective reduction and the oxidation of nitro (nitromethane) or nitrite (n-butylnitrite) compound supported the proposed reaction scheme.