In the present work, ethanol electrooxidation on a Pt(100) electrode modified by different coverage degrees of osmium nanoislands obtained by spontaneous depositions, was extensively studied employing in situ FTIR spectroscopy. A collection of spectra of the ethanol adsorption and oxidation processes was acquired during the first series of a positive potential step, to determine the intermediate species, as well as the main products formed. The spectroscopic results obtained were correlated with conventional electrochemical results obtained by cyclic voltammetry. It was shown that the catalytic activity of Pt(100) for ethanol oxidation increases significantly after osmium deposition and that the mechanistic pathway for this reaction depends directly on the osmium coverage degree. Thus, for low osmium coverage (theta(Os) up to 0.15) the formation of CO2 as an intermediate was favored and hence the full oxidation of adsorbed ethanol to CO2 was increased. For higher osmium coverages (theta(Os), up to 0.33), the higher the coverage is, the more the direct ethanol oxidation to acetaldehyde and acetic acid is favored. For osmium coverage degree of 0.40, the catalytic activity of the electrode for ethanol oxidation decreased. On an almost complete osmium layer (theta(Os), = 0.92) obtained by electrodeposition at 50 mV vs reversible hydrogen electrode, the catalytic activity for ethanol oxidation shows a much lower value.