The effect of cations having high polarizability on the mobilities of coexisting cations as well as of the cations themselves has been studied for Ag+, Tl+ and Cs+ ions. We have previously found that the internal mobilities, u, of alkali ions in molten salts such as nitrates are well expressed by an empirical equation:u = [A/(Vm-V0)] sxp(-E/RT), where Vm- is the molar volume, and A, V0 and E are parameters nearly independent of coexisting cations. In binary melts (M, Ag)NO3, uM except for M=Li+ becomes smaller than expressed by this equation; Ag+ ion has a tranquilization effect on uM. On the contrary, in (M, Tl)NO3, uM becomes higher than according to the equation, which means that Tl+ has an agitation effect on uM. It is found that Cs+, having a high polarizability, also has an appreciable agitation effect.