The development of Direct Ethanol Fuel Cells (DEFCs) is important for obtaining alternative energy converters. Within this objective, there is an interesting topic that is considered fundamental to the development of these devices: the ethanol reaction mechanism in the anode of the DEFCs. Some groups have investigated this process, but still there are many difficulties to achieve a complete understanding of the ethanol reaction mechanism. Using experimentation procedures is difficult to identify the reaction intermediates and the reaction paths, whereas the theoretical investigations are still in development. These facts encourage both experimental and theoretical investigations to understand completely the ethanol reaction process in the DEFCs. Currently, the most commonly investigated catalytic surfaces are Pt-Ru, Pt-Sn, and Pt-Ru-Sn mixtures and some catalytic mixtures that contain nickel, Pt-Ru-Ni and Pt-Sn-Ni. Nevertheless the experimental studies cannot elucidate entirely the reaction intermediates and reaction paths. So to date there are not known satisfactory explanations of the catalytic processes existent in the ethanol adsorption and decomposition processes on different catalytic surfaces. Because of this, the theoretical investigation could help to elucidate the complex reaction mechanism involved in the ethanol reaction in the DEFCs. Considering this, it is carried out in this work the study of the initial steps of the ethanol reaction mechanism on a Pt-Sn catalytic surface. Specifically the potential energy surface (PES) of the adsorption and dehydrogenation steps of ethanol decomposition on a specific catalytic surface (Pt3Sn1 in atomic ratio) is investigated in this work, using self-consistent periodic slab calculations based on density functional theory. This research reveals that ethanol does not have an unique mode of adsorption on this catalytic surface, as well as the dehydrogenation pathway does not only proceed via the ethoxy species formation, but also via the 2-hydroxyethyl species formation. Additionally it is showed that acetaldehyde desorbs in the process of dehydrogenation of ethanol. These results allow to understand in detail the first steps of the ethanol oxidation on a specific catalytic surface, which constitutes a contribution to clarify the problem of selectivity in catalysts for DEFCs.