PROX reaction was studied over an unsupported PtSn catalyst. CO2 and H2O were produced from a H-2 flow containing 1% CO, and 0.4-2% O-2. PtSn pretreated with O-2 and H-2 contained more Pt in the near-surface region and was more active towards CO oxidation than a sample pretreated in H-2 only. In situ XPS was also carried out during PROX reaction. Sn 3d indicated the presence of Sn-oxides, near to the surface region, along with Pt3Sn. Pt 4f peak showed metallic Pt under all conditions. Water and OH enrichment was observed in the O 1s spectrum, taken during PROX reaction. (C) 2011 Elsevier B.V. All rights reserved.