For practical use of molten carbonate fuel cells (MCFCs), Ni shorting due to NiO cathode dissolution during high-pressure operation is still a serious problem. The molten carbonate electrolyte should be more basic in order to reduce the NiO cathode solubility. In order to obtain a lower NiO solubility, the solubility of La oxide was investigated in the Li/Na/K and the Li/Na eutectic carbonates and the effect of the La oxide addition to the molten carbonate on the NiO solubility has also been investigated. The stable phase of La oxide in molten carbonate was found to be La2O2CO3 under P-CO2/P-O2 = 0.7/0.3 atm at 923 K. The solubility of NiO significantly decreased due to La additive in the acid dissolution region that is the operating condition for the MCFCs. The order of the base comparative index on the NiO solubility in the molten carbonate was La much greater than Ca = Sr > Li > Na > K. (C) 2004 Elsevier B.V. All rights reserved.