In this paper, a thermodynamic analysis was carried out to provide useful information about the operation of intermediate-temperature Solid Oxide Fuel Cells (IT-SOFCs) with direct internal reforming (DIR) fuelled by glycerol. A methodology, based on the principle of minimizing the Gibbs energy of a given system, using spreadsheets and the Microsoft Excel's Solver function, was described for DIR operation of the SOFC with oxygen ion conducting electrolyte (SOFC-O(2-)) and proton conducting electrolyte (SOFC-H(+)). The effect of temperature, fuel utilization and type of electrolyte on the equilibrium composition of the anode gas mixture as well as on the boundary of carbon formation has been investigated in the temperature range of 773-1073 K. Based on the results of this thermodynamic study, glycerol can be considered an alternative fuel with suitable characteristics for electricity generation in IT-SOFCs. Operating at carbon-free conditions, between 773 and 1073 K, with a fuel utilization of 99.99% in the anode channel outlet, glycerol-fuelled IT-SOFCs systems attain high theoretical efficiencies in the range of 80.7-89.9% (SOFC-O(2)(-) case) and 90.3-96.7% (SOFC-H(+) case). Regarding the maximum values of the average electromotive force (EMF) and efficiency, it was verified that glycerol exhibits very similar potential for power generation with ethanol. Although glycerol fed SOFC-H(+) is superior to SOFC-O(2)(-) in terms of maximum theoretical efficiency, it should be taken into account that the SOFC-H(+) shows a greater tendency for carbon deposition than does the SOFC-O(2)(-) during the operation. Besides, it was found that decreasing temperature increases the efficiency but also favors carbon formation, for both SOFC-O(2)(-) and SOFC-H(+). When the system runs at 70% of its maximum power and the partial pressure of residual hydrogen in the anode outlet is kept equal to 0.1 atm, the highest efficiency (67%) is achieved by operating an SOFC-H(+) at 823 K. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.