Because chemical compositions in thermodynamic equilibrium for various fuel gases are specified by the ratios among carbon, hydrogen, and oxygen, carbon-hydrogen-oxygen (C-H-O) ternary diagrams are useful to present relevant operational conditions of fuel cells. Such C-H-O ternary diagrams are constructed, describing the carbon deposition region, the fuel gas region, partial pressures of gaseous species including O-2, H-2, and CO, and electromotive force corresponding to the theoretical open circuit voltage of fuel cells. Such ternary diagrams present these operational conditions for any given C-H-O ratios without any additional calculations. (C) 2003 The Electrochemical Society.