The kinetics of isotope exchange in the238U(VI)-233U(VI)-strongly acidic cation exchanger Ostion KS system was studied in the temperature range 275–307K and for total uranium concentration 2.94·10−4–1.75·10−2 mol·l−1 in UO2(NO3)2 solution. The experimental results were evaluated by means of the “two-film mass-transfer model” and by the use of Fick's diffusion equations which have been proved more suitable for the system studied than McKay's equation. The influence of the temperature was evaluated using the Arrhenius equation. The diffusion character of the process follows also from the value of the activation energy (15.12 kJ·mol−1). In comparison with the UO 22+ ↔H+ ion exchange6 the isotope exchange studied is faster and less dependent on temperature (the activation energy is substantially lower).