The importance of recycling and producing rare earth (RE) materials has been highlighted by the critical raw materials study in the European Union (EU). Oxide-fluoride electrolysis is one of the dominant technologies to produce rare earth metals (REMs) or alloys. The solubility of rare earth oxides (REOs) is one of the critical issues during this process. In this research, a model is developed to predict the REO solubility in molten fluorides, based on the systematic analysis of different influential factors and fundamental understanding of the dissolution reactions. The average relative deviation of the model from the experimental data extracted from the literature is 8% for Nd2O3 and 7% for Y2O3, respectively, which is well within the experimental uncertainty. The fitting parameters can indicate the influence of different composition quantitatively. It is also found that the solubility is correlated with the ion charge density of the cations in the molten salts.