Power-generation experiments of solid-oxide fuel cells with Ni/yttria-stabilized zirconia cermet anodes were carried out by changing the water-to-methane ratio in the fuel. To separate the electrochemical reactions occurring on the three-phase boundary of the anode from the steam-reforming reactions, premixed gases corresponding to the composition at thermal equilibrium were used as the fuel. The reactions on the anode were quantitatively clarified at different steam-to-methane ratios by making a mass balance of the elements and electrons and by determining the anode exit gas composition. For dry (i.e., no water) methane fuel, the carbon deposited on the three-phase boundary from the methane mainly reacted with oxide ions (O2-). With increasing steam-carbon (S/C) ratio from S/C = 0.5 to 1, hydrogen became the dominant reactant. The anode potential vs, current density for S/C = 1 agreed with that for hydrogen diluted with argon. At S/C ratios higher than 1.5, CO in the inlet gas reacted with oxide ions.