An analysis of the chemical equilibrium of ethanol/water system, using the stoichiometric method, has been performed. Intermediate compounds and coke formation are analyzed. Ethanol is completely converted to ethylene and/or acetaldehyde. Taking into account the equilibrium constant values of formation and transformation reactions of ethylene and acetaldehyde, both compounds are intermediates in this system. Due to the relevance of carbon monoxide if hydrogen is used as feed of a PEM-type fuel cell, CO concentration in the equilibrium mixture was studied assuming two different scenarios: (a) CO as primary product and (b) CO2 as primary product. These two scenarios lead to suggest different routes for reaching the equilibrium. Thus, the results obtained in this work might help to interpret the experimental results far away from the equilibrium with the aim of elucidating the reaction mechanism. The knowledge of this mechanism is essential in order to minimize the CO formation. The thermodynamic feasibility of coke formation shown in a temperature vs. water/ethanol molar ratio has also been analyzed. The results indicate that if moderate temperatures are used, a molar ratio higher than 3 is required to avoid coke formation. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.