The relative difference in the internal cation mobilities in the molten bin
ary system NdCl3-KCl was measured by countercurrent electromigration method
at 1073 K. Isotherm of the internal cation mobilities was estimated with a
vailable densities at this temperature. The internal cation mobility of Nd3
+ becomes drastically small with decrease of its concentration, which is ex
plained by stable complex formation of NdCl63-.