The ε values in the internal cation mobilities of (Li,K,Cs)NO3 of the eutectic composition (35.2-39.8-25.0 mol%) in the temperature range from 453 to 673 K were measured by Klemm's countercurrent electromigration method, where e is defined as (bi-bj)/ba; ba is the average internal cation mobility and the subscripts i and j refer to any two out of these three cations. The conductivity was measured by a direct current method. From these data and the molar volume calculated from those of the pure salts on the assumption of additivity, the internal cation mobilities, bLi , bK and bCs, have been calculated. The bLi is well expressed by the empirical equation presented for binary alkali nitrates; the negative devia-tion for bK and bCs may be accounted for on the assumption of the tranquilization effect of Li+. The orders of those internal cation mobilities are bCs<bK<bLi at 453-473 K, bCs<bLi<bK at 473 -583 K, and bLi<bCs<bK at 583-673 K. These orders can be interpreted in terms of the dynamic dissociation model previously presented.