The underground disposal of radioactive waste is based upon a multibarrier concept. For long-term performance assessment of radioactive repositories, knowledge concerning the sorption of radionuclides in backfill materials is required. As a part of the multibarrier system for effective isolation of radioactive waste in a repository, Zeolite NaA-X blend was prepared from fly ash, characterized, and evaluated to be used as a backfill material. In this concern, the sorption behavior of Cs+ on the prepared material as a function of pH, initial ion concentration and temperature was studied by batch technique. The sorption isotherm data was interpreted by Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models. The application of the Langmuir isotherm yielded monolayer capacity of 1546 mmol/kg at 298 K while the maximum sorption capacity predicted by D-R isotherm was of 2446 mmol/kg. Thermodynamic parameters for the sorption system were determined at three different temperatures. The enthalpy (Delta H degrees), entropy (Delta S degrees) and free energy (Delta G degrees) of sorption at 298 K were found to be 25.43 kJ/mol, 93 J/mol K and -2.316 kJ/mol, respectively. The positive value of Delta H degrees corresponds to the endothermic nature of the sorption process. The numerical value of Delta G degrees decreases with an increase in temperature indicating that the sorption was spontaneous and more favorable at higher temperatures. (C) 2008 Elsevier B.V. All rights reserved.