In this paper, a thermodynamic analysis of ethanol/water system, using the Gibbs energy minimization method, has been carried out. A mathematical relationship between Lagrange's multipliers and carbon activity in the gas phase was deduced. From this, it was possible to calculate carbon activities in both stable and metastable systems. For the system that corresponds to ethanol steam reforming at very low contact times, composed mainly of ethylene and acetaldehyde, carbon activities were always much greater than unity over the whole temperature range, changing from 1.2 x 10(7) at 400 K to 1.1 x 10(4) at 1200 K. Furthermore, there was practically no effect of the inlet steam/ethanol ratio on carbon activity values. These results indicate that such a system is highly favorable to carbon formation. On the other hand, by considering a more stable system, in order to represent high contact times, it was observed that carbon activities are much lower and depend greatly on the inlet steam/ethanol. ratio employed. Besides, the complete conversion of ethylene and acetaldehyde into other species, such as CO, CO(2), CH(4) and H(2), lowers the total Gibbs energy of the system. By computing carbon activities in experimental systems, it was also possible to explain deviations between thermodynamic analysis and experimental results regarding carbon deposition. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.