For the molten salt systems (Li-Na)N03 and (Na-Cs)N03, relative differences of intei'nal mobilities have been measured with the Klemm method. The internal mobilities, b, are calculated from these data and the available conductivity data. In the mixtures of the former system, &xa is always greater thanIn the latter system, the Chemla crossing point occurs. It is found that bu and &xa in the binary alkali nitrate systems so far investigated are well expressed, except at small molar volumes, by b = {^4/(F-F 0 )} exp(-E/RT) with constant A and E, where V is the molar volume and F° a constant for bu and nearly a constant for The values of bes at equal concentrations of CSNO3 in the systems (Li-Cs)N03 and (Na-Cs)NOs are practically equal. The internal mobilities obtained are discussed in terms of free space, mutual ionic attraction and thermal agitation.