The selective reduction of nitric oxide by propane and propene in an excess of oxygen has been studied over a Pt/ZSM-5 catalyst to elucidate the role of the reducing hydrocarbons in the reaction mechanism. Temperature-programmed reaction (TPR) and transient studies using the temporal-analysis-of-products (TAP) reactor have been performed. Propene is found to be the more efficient reductant compared to propane at T less than or equal to 600 K. Mechanistic studies demonstrate that even in an excess of oxygen carbon-containing species, formed from propene, are adsorbed on the catalyst, which further react with nitric oxide to N-2, N2O and CO2; no such intermediates are formed from propane, giving rise to its far lower reduction efficiency. It is concluded that the main reaction pathway over Pt/ZSM-5 involves a surface reaction between propene-derived adsorbates and NO or, possibly, NO2. Catalytic surface reduction by hydrocarbons, followed by NO decomposition on reduced platinum sites, is proposed as a second, minor, mechanistic pathway at low reaction temperatures (T less than or equal to 600 K).