The sorption behavior of cesium on montmorillonite type clay was studied by using radioactivity measurements. Concentrations of Cs+ ions ranged from 10(-6) to 10(-2) M. Cesium retention reduced with increasing salt concentration which was varied between 10(-4) and 10(-1) M. Selectivity coefficients KCs-Na for the exchange between Cs and Na were calculated for different equivalent fractions of Cs on the solid phase. Using the KCs-Na values, free energy change was found to be 7.8 kJ/mol. The data could be fitted to a Freundlich isotherm, and empirical Freundlich parameters enabled the generation of a site distribution function. By fitting the data to the Dubinin-Radushkevich (D-R) isotherm, a mean energy of sorption of 8.6 kJ/mole was calculated which corresponds to the energy of ion exchange reactions. The values of energy changes calculated by using two different methods were in good agreement.