The isotope separation effect between U(IV) complex and U(VI) ions was investigated in a mixed system of uranyl solution and cation exchange resin containing wranous ions. The columns were filled with the water slurry of the U(IV)-from resin particles. From the variation of the isotope abundance ratio235U/238U of U(VI) in the effuent, it was found that (1) the apparent process of the isotope exchange under study is represented asmore separate (2) the heavy uranium isotope are concentrated preferentially in the U(IV) complex ions; the equilibrium constant, K, is about 1.0005, independently of the experimental conditions. (3) the rate of isotope exchange is extremely sensitive to temperature.