Since sulfur compounds are the major poisons of fuel cell systems, equilibrium concentrations of minor sulfur-based impurities in fuel cell fuels are thermochemically calculated in the temperature range between and . As sulfur-based impurities in fuel cell gases, , elementary sulfur, inorganic sulfur compounds, mercaptans, alkyl (di-)sulfides, thiophenes, and related compounds have been taken into account. Various types of fuels are also considered, including , , CO, , biogas, liquidified petroleum gas, gasoline, kerosene, and diesel fuel. Among the 21 kinds of sulfur-based typical impurities considered, is the most stable sulfur-based species. COS(g) can also coexist, but even in CO-rich gases and in hydrocarbon-based fuels, COS concentration in equilibrium is one order or a few orders of magnitude lower than concentration. Other sulfur compounds, such as at intermediate temperatures and HS(g) and at high temperatures, are also expected to coexist but their concentrations are less than (parts per billion) assuming thermochemical equilibrium.