Ion-exchange-electromigration has been investigated as a method for separating potassium isotopes, and in this case the heavier ion K/sup 41+/ has concentrated on the cathode side. An experimental value 1.045 plus or minus 0.001, for example, has been obtained as the total separation factor defined by (K⁴¹/K³⁹)/sub cathode.//(K⁴¹/K³⁹)/sub pre-exp./. The measurements of isotopic ratio, which is an important factor for isotope separation, have carefully been made by two kinds of mass spectrometer. (auth)