Molar conductivities (Λ) of molten ErCl 3NaCl and ErCl 3KCl systems were evaluated using the data of molar volume, V m, and conductivity, ϰ, which were measured as a function of the mole fraction of ErCl 3 and temperature, by dilatometry and a conventional a.c. technique, respectively. The Λ values of molten ErCl 3 were smaller than those of molten YCl 3, in which a loose disordered network of edge-sharing octahedral units has been reported to exist. The Λ values of ErCl 3NaCl and ErCl 3KCl systems increased with decreasing ErCl 3 concentration, x, and in the range x >4 0.25 the increment in Λ with x was much smaller than that in the range of x < 0.25. In a series of molten rare earth chlorides, Λ values have been found to be affected qualitatively by cationic radii and the types of linkage, that is, corner- or edge-sharing of octahedral units.