The adsorption behavior of Cs ions on a montmorillonite-type clay was investigated in the presence of potassium ions, using a radiotracer technique. The initial concentration of K+ added to the CsCl was between 10(-4) and 10(-1) mol/l. The addition of K+ to the CsCl solution at different concentrations (10(-6)-10(-2) mol/l) reduced the amount of Cs+ adsorbed on clay. The maximum ratio of Cs+ exchanged, calculated from a linearized form of Langmuir plot was in agreement with the ion exchange isotherms of Cs-K ions. Sorption energy evaluated from the graph of corrected selectivity coefficients vs. equivalent fraction of Cs in the solid phase was compared to the energy values obtained from Dubinin-Radushkevich (D-R) isotherms. Freundlich isotherm parameters were used to characterize a site distribution function, which provides information about the affinity ratio of the adsorption sites of Cs+ and K+ ions. The ion exchange isotherm of Cs-K systems exhibited a Langmuir type curve for all K+ concentrations.