AbstractDiffusion coefficients for calcium ions were measured in NaCl + 0.2 mole% CaCl2 and KCl + 0.2 mole% CaCl2 mixed melts, and pure CaCl2 melt, using the capillary method and a Ca45 radio tracer. It was found that the self‐diffusion coefficient for the calcium ion in molten CaCl2 was much smaller than that for cations in some molten alkali chlorides. The impurity diffusion coefficient of the calcium ion in these mixed melts is small in comparison with that of the sodium ion in NaCl melts, in contrast to ionic crystals of the same system. These values are discussed, primarily in terms of a modified theory of liquids. For molten salt, many ionic and molecular species should accompany diffusion processes. When the relative concentration and the diffusion coefficients of each species are appropriate, the observed diffusion coefficients, D*+, can be approximated by a single Arrhenius equation. This applies not only to the self‐diffusion coefficients for molten alkali chlorides but also to the present data for molten CaCl2.It was found that the experimental values for the diffusion coefficient were nearly in agreement with those calculated from the characteristic relations between diffusion and viscosity in liquid structures.