Abstract <p>The sorption and diffusion behavior of cesium in bentonite from the Taganskoe deposit was studied. The sorption isotherms were obtained in a 0.01 M NaClO₄ solution over a wide range of Cs⁺ concentrations from trace levels to 10^–3 M. The cesium sorption is nonlinear and is controlled by the presence of both high- and low-selectivity ion exchange sites. Diffusion properties of the sample were determined in through-diffusion experiments with tritium, allowing the estimation of the effective diffusion coefficient and porosity. Trace amounts of stable Cs⁺ and K⁺ were detected in the supporting electrolyte; they significantly affect the results of both sorption and diffusion experiments. Taking into account these impurities leads to noticeable changes in the sorption isotherms at low concentrations and affects the interpretation of transport parameters. Based on the transport and sorption parameters obtained, the cesium diffusion was modeled using a multicomponent reactive transport approach taking into account the ion exchange and cation competition. Satisfactory agreement between the experimental and modeled data was achieved. The results demonstrate the applicability of thermodynamic simulating to describing the migration of sorbable cations in clay materials.</p>