In the present work, the morphology and composition of osmium deposits formed in submonolayer amount on Pt(1 0 0) and Pt(1 11) single crystal surfaces either by spontaneous deposition or by electrolysis at 50 mV in different deposition times (t(dep)), were studied. The cyclic voltammetric curves for Pt(1 0 0)/Os and Pt(1 1 1)/Os were recorded in the potential range where the oxidation of deposited osmium species occurs, which suggests that the chemical composition of the deposits is a mixture of metallic Os and OsO2. The ratio of each species deposited depends directly on the deposition potential, as well as the surface structure characteristics. The composition of the modified platinum single crystal surfaces has also been estimated by X-ray photoelectron spectroscopy (XPS), whose results agree with those obtained by CV, about the amount of osmium oxide on Pt(1 1 1) to be higher than on Pt(1 0 0). This feature of the Pt(1 1 1)/Os becomes its surface more active for ethanol oxidation at lower potentials than that observed for Pt(1 0 0)/Os, although at high potentials the high presence Of OsO2 is Prejudicial to the catalytic activity of the electrode. The Pt-Os surfaces were also explored by use of the scanning tunneling microscopy (STM) technique in order to image the features of osmium deposits on platinum, after a short and a long deposition time. It was found, by using spontaneous deposition procedure, that the dimensions of osmium deposits islands do not grow with the increase of degree of coverage of osmium on the surface, as well as they keep the monoatomic thickness. (C) 2003 Elsevier Ltd. All rights reserved.