The selective catalytic reduction of NO by propene (HC-SCR) in the presence of excess oxygen over Pt/MCM-41 (Al2O3) and MCM-41 (Al2O3) catalysts has been studied by simultaneous in situ IR-spectroscopy and catalytic activity measurements. IR-spectra of Pt/MCM-41 (Al2O3) and MCM-41 (Al2O3) were compared during the adsorption of the single reactants (NO and C3H6) and during the NO reduction With C3H6 in presence of excess O-2 (NO + C3H6 + O-2 reaction). The adsorption of NO resulted in the formation of surface nitrate species on the Al2O3 support and surface NO species on Pt. Adsorption of propene led to the formation of carboxylate species on Al2O3. Under reaction conditions nitrates, carboxylates, acetates, hydrocarbon fragments, isocyanate, cyanide species and CO were present on the catalyst surface. The surface concentration of the isocyanate species was found to be strongly correlated to the activity of the Pt/MCM-41 catalyst. Besides the formation of isocyanate, cyanide species were also observed on the surface. Both species increased in concentration with increasing oxygen concentration in the feed. The isocyanate species were found to be a reaction intermediate during the NO reduction, whereas the cyanide species were stable against further reaction. For the cyanide formation, a reaction pathway via isocyanate. by abstraction of an oxygen atom by propene was observed.