Preferential oxidation of CO (PROX) in hydrogen-rich mixtures was studied over alumina-supported Pt and Pt-Sn catalysts, which exhibited a well-defined metal particle size. The reaction rate was much faster over Pt-Sn/Al2O3 than over Pt/Al2O3. Significantly different apparent activation energies and oxygen reaction orders were found for both samples. Whereas the CO PROX kinetics over Pt/Al2O3 could be described by oxygen adsorption as being rate-determining step, this was not the case for Pt-Sn. In situ Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS) showed that Pt-Sn, initially under a mixture of 1% CO/80% H-2, segregated upon the introduction of 2% O-2, even at temperatures where the alloy was stable under reducing conditions. For the CO PROX reaction over Pt-Sn/Al2O3 a bifunctional mechanism is proposed with CO and H being adsorbed on the platinum sites and oxygen being channeled from neighboring SnOx sites. This work suggests a Mars-Van Krevelen type of mechanism for the SnOx, site, in agreement with the low value of 0.2 for the oxygen reaction order over Pt-Sn. (C) 2015 Elsevier B.V. All rights reserved.