In this study, the adsorption and the reactive properties of alpha-beta-unsaturated aldehydes are studied by means of density functional calculations (DFT) on two well-defined Pt-Sn alloy surfaces, p(2 x 2) Pt3Sn(111) and (root3 x root3) R30degrees Pt2Sn(111). First the electronic structure of the bulk alloys is determined: a charge transfer occurs from Sn to Pt, the work function decreases, and the d-band center is shifted away from the Fermi level. Then various adsorption structures of acrolein (propenal), crotonaldehyde (2-butenal), and prenal (3-methyl, 2-butenal) are considered on the two alloys. The results are compared with those obtained on Pt(111). A large decrease of the adsorption energies is observed for the alloys. For acrolein, there is almost no change in the best adsorption modes, the adsorption through the C=C bond being predominant. This explains why the selectivity of the acrolein hydrogenation is not changed much when Pt-Sn alloys and Pt are compared. For prenal on the contrary, only the atop mode is stable on the alloys, whereas a structure parallel to the surface is the most stable one on Pt. This explains the modified reaction selectivity with a larger amount of unsaturated alcohol in the products in the case of the Sn alloys. (C) 2003 Elsevier Inc. All rights reserved.