Using thermochemical data of ca. 300 compounds consisting of carbon, hydrogen, and oxygen, the amounts of equilibrium products have been calculated for various fuel cell fuels including alkanes (CH4, C3H8, C8H18, C12H26), alcohols (CH3OH, C2H5OH, C3H7OH), alkenes, alicyclic hydrocarbons, and dimethyl ether, as well as for other hydrocarbon-containing fuels such as biogas and coke oven gas, in the temperature range between 100 and 1000degreesC. It has been revealed that the major constituents in typical fuel cell gases in thermodynamic equilibrium are H-2(g), H2O(g), CO(g), CO2(g), CH4(g), and C(s). Possible minor constituents are specified while their concentrations were negligibly low. Derived are the minimum amounts of H2O and CO2 for reforming and O-2 for partial oxidation, thermodynamically essential to prevent carbon deposition. (C) 2003 The Electrochemical Society.