The removal of cesium from concentrated aqueous solutions into Ca/Mg-bentonite for a wide range of bentonite-to-water (m/V) ratios was studied experimentally and theoretically. Using the batch technique, the equilibrium of Cs uptake was measured. The nonlinear character of cesium sorption substantially influenced by the m/V ratio was observed. The experimental data were evaluated using the multicomponent Langmuir isotherm and an ion-exchange model based on the ion-exchange reaction between Cs+ and M2+ (Ca2+/Mg2+) initially sorbed on bentonite. Constants k(1,Cs) = 0.521 mmol center dot g(-1), k(2,Cs) = 968 L center dot mol(-1), and k(2,M) = 592 L center dot mol(-1) were obtained for Cs uptake described by multicomponent Langmuir isotherm. For the ion-exchange model, the thermodynamic equilibrium constant K = 75.5 mL center dot g(-1) with a standard deviation of s(K) = 17.4 mL center dot g(-1) was determined. Using the t test, the calculated data of multicomponent Langmuir and ion-exchange isotherms were fit to experimental data, and the best agreement was obtained for the ion-exchange model. The results show that Cs uptake by bentonite could be substantially decreased in systems with a high bentonite-to-water (m/V) ratio as a consequence of the presence of desorbed divalent cations in the liquid phase.