The data on internal cation mobilities, u, in the molten binary nitrate systems (Ag, M)NO3 and (Tl, M)NO3 (M=alkali metal ion) previously obtained by us have been re-examined. In binary alkali nitrates Ir is known to be well expressed by an empirical equation u={A/(V-m-V-0)} exp (-E/RT), where V-m is the molar volume; while A, V-0, and E are constants nearly independent of the cations with which they are mixed, It is found that in melts containing Ag+, u(M), for the M deviates negatively from this Equation, and that with Tl+, u(M) deviates positively. Thus, Ag+ and Tl+ both with relatively high polarizabilities, have opposite effects on u(M), that is a tranquilization effect and an agitation effect, respectively The reason has been interpreted in terms of the high polarizabilities and the difference in the sizes of Ag+ and Tl+.