Bimetallic catalysts based on Pt and Sn are used for commercial alkane dehydrogenation processes and also exhibit promising activities for the preferential oxidation of CO [1,2]. It was recently shown that the structure of Pt-Sn particles dramatically evolved when going from a reducing atmosphere to one containing O2 [1,3]. Pt-Sn alloyed phases are readily destroyed in the presence of O2, leading to Pt highly dispersed in SnOx domains [1,3]. We have further investigated the structure of Pt-Sn nanoparticles under reducing conditions, always free of O2. Surprisingly, we observed by in situ diffuse reflectance IR spectroscopy (DRIFTS) that the alloy phase was not stable under (i) H2-free CO at any temperature or (ii) CO/H2 mixtures at temperatures below 175°C. The DRIFTS data (figure 1.left) show that bands typical of CO adsorbed on Pt-Sn phases (ca. 2046 cm-1) are readily replaced with bands associated with Pt phases (> 2060 cm-1). Even more surprising, X-ray photoelectron spectroscopy (XPS) revealed that the proportion of oxidized Sn increased from about 38% to 69% following exposure to pure CO (Fig. 1.middle and right). The XPS results were confirmed by in situ X-ray absorption spectroscopy, in particular XANES analysis carried out at the Sn K edge (data not shown). These observations could be rationalized by the known, albeit very slow, ability of Pt to dissociate CO [4] and we propose that the oxygen atoms formed readily reacted with metallic Sn to form a SnOx phase, leading to Pt-Sn destruction, alongside carbon deposition. A similar scheme had been proposed to explain silver oxidation in the presence of CO at room temperature [5]. These data highlight an important phenomenon affecting metal and alloy stability under CO, which is of strong interest to both heterogeneous catalysis and electrochemical communities. [1] A. Moscu, L. Veyre, C. Thieuleux, F.C. Meunier, Y. Schuurman, Catal. Today, 258 (2015) 241-246. [2] A. Moscu, Y. Schuurman, L. Veyre, C. Thieuleux, F.C. Meunier, Chem. Commun. 50 (2014) 8590-8592. [3] W.D. Michalak, J.M. Krier, S. Alayoglu, J.Y. Shin, K. Komvopoulos, Z. Liu, G.A. Somorjai, J. Catal. 312 (2014) 17-25. [4] Y. Iwasawa, R. Mason, M. Textor, G.C. Somorjai, Chem. Phys. Lett., 44 (1976) 468-470. [5] K. Bechoux, O. Marie, M Daturi, G. Delahay, C. Petitto, S. Rousseau, G. Blanchard, Cat. Today 197 (2012) 155–161