Single-chamber solid oxide fuel cells (SC-SOFCs) rely on the selectivity of the anode and cathode materials for either the partial oxidation reaction of a hydrocarbon with oxygen from air (anode) or the oxygen reduction reaction (cathode). The observed open-circuit voltages (OCVs) of up to 0.9 V measured in SC-SOFCs suggest a very low oxygen partial pressure to be present at the anode side of the electrolyte while at the cathode it is more than 15 orders of magnitude higher but lower than in the gas phase. As lower boundary of the oxygen partial pressure in the anode the partial pressure of oxygen that is given by the equilibrium state of the gas mixture can be assembled. The initial nonequilibrium gas mixture of a fuel and air will react toward the equilibrium composition, which in turn is only a function of temperature and the initial gas composition. We present here the calculation of thermodynamic equilibrium for methane-air mixtures relevant in SC-SOFCs. Furthermore, we show that the calculation can describe qualitatively the observed OCVs and give guidelines to suitable gas mixtures for enabling high electric loads in SC-SOFCs. (c) 2006 The Electrochemical Society.