The solubility of rare earth metal oxides and their effect on the NiO solubility have been discussed to stabilize the cathode of molten carbonate fuel cells. The solubility of Ho, Yb, and Nd oxides were 4.4 x 10(-4) 3.4 x 10(-4), and 1.3 x 10(-3) (mole fraction) at 923 K, respectively. The solubilities of NiO in (Li-0.52/Na-0.48)(2)CO3 with the saturated Ho, Yb, and Nd were 1.57 x 10(-5), 1.41 x 10(-5), and 9.5 x 10(-6), respectively. Among these three, Nd, which has the highest solubility in the carbonates, reduced the NiO solubility most; although, the La reduced the NiO solubility more than Nd. The logarithm of the solubility of the rare earth metal oxides has a linear relation to the Coulomb force ratio between the rare earth metal and the alkaline metal. Following this relation, the La should have the highest solubility among all the lanthanides. The basicity which NiO solubility closely relates has a linear relationship to the Coulomb force parameter of the melts. Based on these two models, the La would be the best additive to reduce the NiO solubility in Li/Na eutectic carbonate melt, among all the lanthanides. (c) 2006 Elsevier B.V. All rights reserved.