In the present work, osmium was deposited in submonolayer amounts on Pt(1 0 0) and Pt(1 1 1) single crystal surfaces either by spontaneous deposition or by electrodeposition, at various deposition times. The Pt-Os surfaces were explored using ex situ scanning tunneling microscopy to image the deposition characteristics of osmium on Pt(1 1 1). It was found, using the spontaneous deposition procedure that osmium clusters with diameters between 5 and 7 nm and monatomic thickness were formed. A homogeneous distribution of the islands on the platinum terraces was found, with no observed preferential deposition on step edges or surface defect sites. The catalytic activity of the modified electrodes for ethanol oxidation depends directly on the method of preparation of the surface. The Pt(1 0 0)/Os surface prepared by an electrodeposition procedure at 50 mV, under an osmium coverage in the range of 0.4-0.6 monolayer, showed the highest activity for the organic oxidation, although the Pt(1 I 1)/Os surface exhibited higher activity than Pt(I 0 0)/Os at low potentials. The electrochemical experimental results support strongly the bifunctional mechanism of the enhanced ethanol oxidation on Pt-Os electrodes. Thus, the improvement of catalytic activity of a Pt-Os electrode occurs due to the adsorption of oxygenated species on this surface at lower potentials than those observed for platinum. (C) 2003 Elsevier B.V. All rights reserved.