Relative differences in internal mobilities for the binary molten system (Li,Cs)Cl were measured by a countercurrent electromigration method at 913 K and 973 K. The internal mobilities of Li+ and Cs+ were calculated from these data and from those available for the electrical conductivities and densities of the mixtures. The Chemla effect occurs, i.e. the isotherms of the mobilities plotted as a function of concentration cross each other. The internal mobilities u(c)(i) are found to be well described by the expression u(c)(i)=[A/(V-m-V-0)] exp(-E/RT), where V-m is the molar volume and A, V-0 and E are constants which are different at two ranges of V-m. The present data are compared with those obtained previously by the emf method.