An experimental evaluation, under realistic conditions, of the catalytic properties of a Pt3Sn(1 1 1) model catalyst in CO oxidation under excess of hydrogen is reported in this study. From a structural point of view, two different surface terminations are available in this alloy, viz. (2 x 2) and (root 3 x root 3)R30 degrees, differing by their tin concentration and Pt-Sn local structural arrangements in the uppermost layers, thus offering specific active sites for the reaction to proceed. In this study, mass spectrometry and polarization modulation infrared reflection absorption spectroscopy measurements (PM-IRRAS) have been performed at near ambient pressure on both surfaces of the alloy and compared to those obtained on Pt(1 1 1) used as a reference. The presence of hydrogen promotes the rate of CO oxidation on the three surfaces. The influence of tin is remarkable, increasing the catalytic activity of Pt atoms by one order of magnitude. With the exception of CO, no other species such as formyls or carbonates are observed on the surface by IR spectroscopy in the range of temperature (293-425 K) investigated. (C) 2010 Elsevier Inc. All rights reserved.