With the countercurrent electromigration method, the ionic mobility differences of Li+and K+in molten binary electrolyte LiCl‐KCl have been measured over a widely concentration and temperature range. Except at a region of high concentration of LiCl, the mobility of the large cation,K+(bk), is greater than that of the smaller Li+(bu) cation. The crossing points of the isotherm mobility shifts with the temperature with a higher concentration of LiCl. When the Coulombic attraction force is expected to be the dominant factor for ruling the cation mobilities, bothbuandbKhave been found to be well expressed by the empirical equationb = [A/(V— Vo) ]exp(—E/RT), whereA, VoandEare characteristic constants of the cation,Vis the molar volume of mixtures, andRis the gas constant.