The influence of H2S fuel impurity on power generation characteristics of solid oxide fuel cells (SOFCs) has been analyzed by measuring cell voltage at a constant current density, as a function of H2S concentration, operational temperature, and fuel gas composition. Reversible cell voltage change was observed around 1000 degrees C, while fatal irreversible degradation occurred at a lower operational temperature, at a higher H2S concentration, and at a lower fuel H-2/CO ratio. Sulfur tolerance of SOFCs was improved by using Sc2O3-doped ZrO2 instead of Y2O3-doped ZrO2 as electrolyte and/or as electrolyte component in the anode cermets. It has been found that H2S poisoning consists of at least two stages, i.e., an initial cell voltage drop within a short time period to a metastable cell voltage, followed by a gradual larger cell voltage drop. Possible H2S poisoning processes are discussed. (c) 2006 The Electrochemical Society.