In an electrolytic reduction process developed to treat spent oxide fuels, alkali and alkaline earth metals (AM and AEM) are dissolved in a molten salt. The dissolution characteristics of AM and AEM are used to selectively separate them from the spent oxide fuel by using a porous MgO filter and, accordingly, to reduce the heat load and the radioactivity from a metal product of the electrolytic reduction process. In this work, the concentration changes of Cs, Sr, and Ba in molten LiCl were measured and the mass transfer behavior was correlated with a diffusion model. The effects of the Li2O concentration on the mass transfer were investigated and the applicability of the porous MgO filter for the selective separation was demonstrated by analyzing the chemicals in the molten salt phase and in the filter.