Reaction characteristics of an anode and cathode were investigated by overpotential analyses in 100 cm(2) class molten carbonate fuel cells. For the overpotential analysis, a reactant gas addition (RA) method was developed in this work, wherein reactant flow rate and partial pressure were used as parameters by the addition of a reactant to an electrode. RA represented these effects on the electrode reactions in the form of overpotential. RA revealed that the anodic overpotential was comprised of each overpotential due to the gas-phase mass-transfer resistance of the reactants, H-2, CO2, and H2O, thus depending on the flow rate of the species. It also indicated that the anode reaction was a severe mass-transfer control process of CO2 and H2O species compared with H-2 at normal operation conditions. RA showed that cathodic overpotential was mainly due to a deficiency of O-2 species in the carbonate electrolytes and depended on the O-2 and CO2 partial pressures. The cathodic overpotential also showed flow rate dependence at high utilizations. RA revealed reaction characteristics at the anode and cathode, respectively; the anode is a mass-transfer control process in the gas phase, whereas the cathode is mainly a mass-transfer control process in the liquid phase at normal operation conditions. (C) 2004 The Electrochemical Society.