Tin deposited on platinum in underpotential conditions was studied by voltammetric and X-ray photoelectron spectroscopy (XPS) techniques. Tin induces a strengthening of bonds between platinum and adsorbed hydrogen or oxygen. The electrochemical and catalytic properties of the platinum substrate are irreversibly modified by tin deposition. Although it is possible to "regenerate" the first monolayers of platinum by voltammetric cycling (0-1.5 V/RHE), some of the tin deposit which diffuses into the platinum bulk (at a depth of 200 angstrom) cannot be desorbed. This bulk tin is found in the oxidized state (Sn(II)), and probably forms a complex compound with platinum and oxygen. In this case, adsorption of Sn(IV) on the regenerated platinum surface with partial or no reduction occurs. This complex behaviour particular to tin deposited underpotentially on platinum could explain conflicting reports of electrocatalytic effects in the literature.