A detailed thermodynamic analysis is undertaken for carbon formation in a solid oxide fuel cell (SOFC) with a direct internal reformer (DIR) fuelled by methanol. Two types of fuel cell electrolyte, i.e. oxygen-and hydrogen-conducting, are considered. Equilibrium calculations are performed to find the range of inlet steam methanol (H2O:MeOH) ratio where carbon formation is thermodynamically unfavourable in the temperature range of 500 to 1200 K. The key parameters that determine the boundary of carbon formation are temperature, type of solid electrolyte, and the extent of the electrochemical reaction of hydrogen. The minimum H2O:MeOH ratio for which carbon formation is thermodynamically unfavoured decreases with increasing temperature. Comparison between the two types of electrolyte reveals that the hydrogen-conducting electrolyte is impractical for use given the tendency for carbon formation. This is mainly due to the water formed by the electrochemical reaction at the electrodes. (C) 2004 Elsevier B.V. All rights reserved.