The reduction of nitric oxide by propene in the presence of excessive oxygen over Pt/ZSM-5 has been investigated by in situ diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) over a wide range of temperatures. It has been found that the main surface species formed during the reaction at low temperature (less than or equal to 473 K) are nitro and nitrito organic compounds on both Pt/ZSM-5 and NaZSM-5, while at higher temperature (greater than or equal to 473 K) carbonyl-carbonate-carboxylates are formed. Surface species formed under exposure to various mixtures of gases have also been studied. It has been further found that the reaction between nitric oxide and propene over Pt/ZSM-5 can produce carbon monoxide at 473 K. On the basis of these results, the direct-decomposition mechanism is proposed. It is concluded that neither the nitro and nitrito organic compounds nor isocyanate species are the most abundant species in this reaction on Pt/ZSM-5.