The operation of Ni-YSZ anode supported SOFCs on methane fuel is described. Power densities as high as 0.52 W/cm(2) at 700 degrees C and 1.27 W/cm(2) at 800 degrees C were observed. Open circuit voltages (OCV) increased with increasing temperature to nearly 1.2 V, in good agreement with equilibrium calculations. Measurements of direct-methane SOFC stability showed that at lower temperatures, <= 700 degrees C, stable operation without coking occurred over a wide range of current densities. At higher temperatures, increasingly large currents were required to avoid cokin and cell failure. In electrochemical impedance spectra, two electrode arcs were observed with the higher frequency arc increasing in size when the fuel was switched from hydrogen to methane. Mass spectrometer measurements showed that the expected reaction products-H-2,H- H2O, CO, and CO2-all increased with increasing cell current density. The dominant products at 800 degrees C were H-2 and CO, in agreement with thermodynamic predictions. Reasons for stable coke-free direct methane operation are discussed. (c) 2005 Elsevier B.V. All rights reserved.