Bimetallic Pt-Sn/C catalysts have been characterized by TPR, CO chemisorption, X-ray photoelectron spectroscopy (XPS) and electron microscopy techniques (TEM and HRTEM). Characterization has been carried out on the samples after reduction and after exposure to air at room temperature. On the surface of the reduced catalysts platinum is in the metallic state whereas tin is mainly in an oxidized state and only a minor fraction is present in a zero-valent state, likely in combination with platinum. On the most tin-rich catalysts, PtSn alloy particles have been identified. Electron microscopy has revealed that the size and the morphology of the metal particles are dependent on the tin loading. Moreover, the combination of the techniques used has shown that, after exposure to air, surface oxidation of zero-valent metal species occurs but their bulk structure is retained. On the basis of a comparison between the characterization data here reported and the catalytic activity measured in the hydrogenation of alpha,beta-unsaturated aldehydes over these catalysts it is suggested that the active sites for the selective hydrogenation reaction are made of metallic platinum decorated by tin, most likely in an ionic form.