Single-chamber solid oxide fuel cells (SC-SOFCs) incorporating thin-film Sm0.15Ce0.85O1.925 (SDC) as the electrolyte, thick Ni+SDC as the (supporting) anode and SDC+BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3−δ ) as the cathode were operated in a mixture of methane, oxygen and helium at furnace temperatures of 500–650°C. Because of the exothermic nature of the oxidation reactions that occur at the anode, the cell temperature was as much as 150°C greater than the furnace temperature. Overall, the open circuit voltage was only slightly sensitive to temperature and gas composition, varying from ∼0.70 to ∼0.78V 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 ∼1:1. At a furnace temperature of 650°C (cell temperature ∼790°C), a peak power density of 760mWcm−2 was attained using a mixed gas with methane, oxygen and helium flow rates of 87, 80 and 320mLmin−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.