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 Y2O3, CeO2, and Gd2O2CO3 in the Li/Na eutectic carbonate were 4.7 x 10(-5) (Y: 9.5 x 10(-5)), 9.5 x 10(-8) (Ce: 9.5 x 10(-8)), and 1.8 x 10(-3) (Gd: 9.0 x 10(-4)) (mole fraction) at 923 K and pO(2)/pCO(2) = 0.3/0.7 atm. The NiO solubility was reduced by the rare-earth oxide addition. However, the effects of the Y2O3, CeO2, and Gd2O2CO3 addition were much smaller than that of La2O2CO3. This difference would be caused by the low solubilities of Y2O3, CeO2, and Gd2O2CO3 compared with that of La2O2CO3 [La: 1.0 x 10(-2) (mole fraction)]. The relationship between the solubility of NiO and the composition of the carbonate melt with a cation additive has also been discussed. © 2005 The Electrochemical Society. [DOI: 10.1149/1.1904985] All rights reserved.