This paper develops a model for the study of solid oxide fuel cell (SOFC) electrical efficiency. The chemically reacting flow model considers the coupled behavior of fluid mechanics, thermodynamic equilibrium, and transport of electroactive species through the electrolyte. Fuel cells are treated as plug flow tubular reactors with no upstream diffusion. Minimization of free energy is used to calculate gas composition along the length of the fuel channel without specifying a set of reactions. The model serves to place an upper bound on electrical efficiencies. The usefulness of the model is demonstrated by simulating SOFC electrical efficiency as a function of operating voltage for in situ reforming of four different fuel mixtures, including steam reforming and partial oxidation of CH4, syngas, and pure CH4. (c) 2005 Published by Elsevier B.V.