Abstract Internal cation mobility ratios in molten (Ca, Ba)Cl2 have been measured by the Klemm method for χBa (mole fraction of barium chloride) = 0.25 -0.51 at 973 K and χBa = 0 -0.61 at 1073 K. From these and available data on the densities and conductivities the internal cation mobilities b have been calculated, assuming additivity of the molar volume. Although bCa in pure CaCl2 is greater than bBa in pure BaCl2, bCa is smaller than bBa in all the investigated mixtures at the respective temperatures. With increasing molar volume at constant temperature both bCa and bBa decrease. Since the profile of the isotherms is similar to that for the binary monovalent chloride systems so far studied, monoatomic ionic species are assumed to carry the current also in the alkaline earth chloride melts.