Abstract: This paper deals with the exchange reactions of U-isotopes between an aqueous solution of uranyl salt and a bed of cation exchange resin particles in displacement chroma-togram. It has been proved by experiment that when the exchange reaction of U-isotopes of uranyl ion between two phases is carried out, the isotope of U-235 is enriched in the liquid phase; and then the exchange reaction between the uranyl ions and the complexes (UO2A) in the liquid phase takes place and the isotope of U-235 is enriched in the UO2A. It has been shown that two isotope effects mentioned above are superimposed in the separation process. Moreover, a simple model and an expression of the superimpo-sition are given, which indicates that the slight difference between U-235 and U-238 in their ability to form the complexes is the main reason for the separation of the uranium isotopes.