The reaction mechanism for the selective catalytic reduction (SCR) of nitric oxide (NO) over Pt/Si-MCM-41 catalyst has been studied by XPS and in-situ Fourier transform infrared (FTIR) spectroscopies as well as TPR. The XPS results indicate that Pt species are reduced during the reaction of NO-SCR in the presence of excess oxygen. The reduced Pt species are found not to be completely oxidised after they have been in contact with NO or O-2 at the reaction temperature. FTIR results indicate that CO is one of the reaction intermediates taking part in the selective reduction of NO. The possible reaction mechanism for the selective catalytic reduction of NO over Pt/Si-MCM-41 catalyst is proposed.