A novel solid oxide fuel cell (SOFC) system, which does not need to separate the supply of fuel and oxidant gases and generates electric power as well as chemicals, was studied. The fuel cell consisted of PtBaCe0.8Y0.2O3-alphaAu, in which two electrodes were exposed to the same mixture of CH4 and air. Electromotive forces (EMFs) were about 700 similar to 800 mV at operating temperatures between 750 and 950 degrees C, and terminal voltages were about 420 mV with discharge a current density of about 400 mA cm(-2) (0.17 W cm(-2)) at 950 degrees C and 350 mV with 75 mA cm(-2) (0.03 W cm(-2)) at 750 degrees C. The working mechanism of the fuel cell was clarified to be based on the difference in catalytic activity for the partial oxidation of methane between two electrode materials : the Pt catalyzes the partial oxidation of methane to form hydrogen and carbon monoxide, while the Au is inactive to this reaction. Therefore, the Pt acts as a fuel electrode, while the Au acts as an oxygen electrode at which electrochemical reduction of oxygen takes place on discharging the cell.