The kinetics of sorption of uranium(VI) from aqueous uranyl nitrate solutions on strongly acidic cation exchanger Ostion KS was studied. The influences of the concentration of UO2(NO3)2 in the range 10−3–10−1M and of the temperature (275 K–313 K) were examined. The results were evaluated by means of “two-film mass-transfer model” modified for systems with non-linear equilibrium isotherms. It has been found that the applicability of the kinetic model used depends significantly on the solution cencentration. The effects of temperature was evaluated by the Arrhenius equation. The activation energy amounts to 23.36 kJ/mol, which corresponds to the assumption on the diffusion character of the ion exchange process.