The mechanism of selective reduction of NO by propylene has been examined over Rh/TiO2 catalysts, in the absence and presence of oxygen in the feed, employing FT-IR and transient-MS techniques. It has been found that no direct interaction between NO and propylene is required for the reaction to proceed. Reduction of NO requires the presence of reduced rhodium sites and the role of the hydrocarbon is to remove adsorbed oxygen and restore the catalytically active sites. Oxidation of propylene and removal of atomic oxygen occurs via intermediate formation of acrolein and acrylate species, which eventually decompose to yield CO and carboxylates on the catalyst surface. The mechanism of activation of propylene does not depend on the nature of the oxidant (NO or O-2) and is the same in the absence and presence of oxygen in the feed.