The nature of the surface species on Pt/Al2O3 for the SCR of NO by C3H6 under excess oxygen has been investigated. An experiment consisting of three steps shows that there are at least three types of surface species occurring on the catalyst surface. However, a reactivity test and an investigation of surface species after adding a second metal (Fe, Sn, Co, Cr or W) to Pt/Al2O3 indicate that only (ClHmOn)(HT) species play an important role in NO reduction at a low operating temperature.