Kinetic measurements on preferential CO oxidation in a H2-rich atmosphere (PROX) over a bimetallic, carbon supported PtSn catalyst reveal a high activity and selectivity already at low temperatures (0–80°C), superior to a commercial Pt/Al2O3 system. The selectivity, though steadily decreasing with temperature, is remarkably high, 85% at low temperatures around 0–20°C, and even at 120°C it is, at 45%, still higher than that of standard Pt catalysts. The observation that CO desorption is not rate limiting and that the selectivity decreases with increasing temperature, can be explained in a mechanistic model involving separation of the reactant adsorption sites (bifunctional surface), with competing CO and hydrogen adsorption on Pt sites/areas and oxygen adsorption predominantly on Sn sites and SnOx islands on/adjacent to the active PtSn particles. The reaction takes place in a bifunctional way at the perimeter of these islands or by invoking a spill-over process. This model is supported by CO temperature-programmed desorption (TPD), in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), and x-ray photon spectroscopy (XPS) measurements, which indicate that under reaction conditions the surface CO coverage on the metallic particles is high, but decreases with temperature, and that only part of the Sn is reduced, included in PtSn alloy particles, while another part is in an oxidic state, forming SnOx islands on and presumably also beside the active particles. Its excellent performance makes PtSn an interesting catalyst for fuel gas purification in low temperature polymer electrolyte membrane fuel cell technology (PEM-FC).