Solid oxide fuel cells (SOFCs) with thin yttria-stabilized zirconia (YSZ) electrolytes on porous Ni-YSZ anodes were successfully operated with humidified methane and natural gas. For both fuels, single-cell tests were carried out with low fuel utilizations in order to approximate the conditions expected near the fuel inlet in a direct methane SOFC stack. Open circuit voltages (OCV) increased from approximate to 1.1 to 1.17 V, and power densities increased from approximate to 0.1 to 1.0 W/cm(2), as the operating temperature was increased from 600 to 800 degreesC. Comparisons of open circuit voltages with Nernst potentials suggested partial oxidation of solid C as the main anode reaction. SOFCs were operated in methane for >90 h at 700 degreesC and 0.6 V, yielding a stable power density of approximate to 0.35 W/cm(2). Very little carbon was detected on the anodes, suggesting that carbon deposition was limited during cell operation.