We have developed a general mathematical model to represent the membrane-electrode assembly (MEA) of fuel-cell systems. The model is used to analyze the effects of various polarization resistances on cell performance. The model accommodates arbitrary gas mixtures on the anode and cathode sides of the MEA. Moreover, it accommodates a variety of porous electrode and electrolyte structures. Concentration overpotentials are based on a dusty-gas representation of transport through porous electrodes. The activation overpotentials are represented using the Butler-Volmer equation. Although the model is general, the emphasis in this paper is on solid-oxide fuel-cell (SOFC) systems for the direct electrochemical oxidation (DECO) of hydrocarbons. (C) 2003 Elsevier Science B.V. All rights reserved.