Single-chamber solid oxide fuel cells (SC-SOFCs) incorporating thin-film SM(0.15)Ceo(0.85)O(1.925) (SDQ as the electrolyte, thick Ni + SDC as the (supporting) anode and SDC + BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3-delta) as the cathode were operated in a mixture of methane, oxygen and helium atfumace temperatures of 500-650 degrees C. Because of the exothermic nature of the oxidation reactions that occur at the anode, the cell temperature was as much as 150 degrees C greater than the furnace temperature. Overall, the open circuit voltage was only slightly sensitive to temperature and gas composition, varying from similar to 0.70 to similar to 0.78 V over the range of conditions explored. In contrast, the power density strongly increased with temperature and broadly peaked at a methane to oxygen ratio of similar to 1:1. At a furnace temperature of 650 degrees C (cell temperature similar to 790 degrees C), a peak power density of 760mWbm-2 was attained using a mixed gas with methane, oxygen and helium flow rates of 87, 80 and 320 mL min(-1) [STP], respectively. This level of power output is the highest reported in the literature for single chamber fuel cells and reflects the exceptionally high activity of the BSCF cathode for oxygen electro-reduction and its low activity for methane oxidation. (c) 2006 Elsevier B.V. All rights reserved.