For the commercial application of molten carbonate fuel cells (MCFCs) under high-pressure operation, Ni shorting may be a problem. To improve the MCFC cathode stability, the solubility of rare-earth oxides and the effects of the addition of these rare-earth oxides to the molten carbonates have been studied. The solubilities of , and in the eutectic carbonate were , and (mole fraction) at 923 K and . The NiO solubility was reduced by the rare-earth oxide addition. However, the effects of the , and addition were much smaller than that of . This difference would be caused by the low solubilities of , and compared with that of [ (mole fraction)]. The relationship between the solubility of NiO and the composition of the carbonate melt with a cation additive has also been discussed.