Hydrogen sulfide, usually considered to be an anode catalyst poison, strongly promotes methane oxidation over sulfur resistant catalysts. The role of H2S in oxidation of sour CH4 feed over a sulfur resistant SOFC anode catalyst at 850 degrees C has been determined through thermodynamic and experimental studies. The anode material was an intimately mixed 50/50 mass ratio composite of La0.4Sr0.6TiO3-delta and YSZ. The power density improved dramatically from 2 mW cm(-2) with pure CH4 as fuel to more than 450 mW cm(-2) for CH4 containing 20% H2S. However, H2S was not the only fuel converted, and its interaction with CH4 was the key factor in enhancement of performance. The anode effluent gas mixture included H2S, SO2, H-2 and CS2 consistent with thermodynamic predictions. The role of H2S in the CH4 oxidation mechanism was determined, and showed that CH4 and H2S must be considered together as a fuel and not as two separate fuels. The types of electrochemical reactions that occurred in the fuel cell were strongly dependent on potential. (C) 2011 Elsevier B.V. All rights reserved.