Three different planar anode supported solid oxide fuel cells (SOFC) were tested with hydrogen, with autothermally prereformed natural gas from which sulfur was removed, and with autothermally prereformed natural gas that contained sulfur. The cells were obtained from Forschungszentrum Jülich (FZJ), Energy research Centre of the Netherlands (ECN), and HTceramix SA (HTc). All cells were so called Real-SOFC first generation cells. Cell polarizations were first measured with hydrogen, followed by a test (, ) with a selected fuel, and finally cell polarizations were measured with hydrogen. When hydrogen was used as the fuel in the test, the performance for all cells was comparable and no degradation was observed. All cells underwent an initial deactivation process when reformate fuels were used but their cell voltage stabilized during the first . All cells also showed deactivation after the reformate tests when the area specific resistance values were compared to the values obtained from the hydrogen tests. The deactivation was comparable between the sulfur-free and sulfur-rich reformate tests. Sulfur-rich reformate, however, caused oscillation in cell voltages as the sulfur level in natural gas was not constant.