Adsorptive behavior of natural clinoptilolite-rich zeolite from Balikesir deposites in Turkey was assessed for the removal of uranium from aqueous Solutions. The uranium uptake and cation exchange capacities of zeolite were determined. The effect of initial uranium concentrations in solution was studied in detail at the Optimum conditions determined before (pH 2.0, contact time: 60 minutes, temperature: 20 degreesC). The uptake equilibrium is best described by Langmuir adsorption isotherm. Some thermodynamic parameters (DeltaHdegrees, DeltaSdegrees, DeltaGdegrees) of the adsorption system were also determined. Application to fixation of uranium to zeolite was performed. The uptake of uranium complex on zeolite followed Langmuir adsorption isotherm for the initial concentration (25 to 100 mug/ml). Thermodynamic values of DeltaGdegrees, DeltaSdegrees and DeltaHdegrees found show the spontaneous and exothermic nature of the process of uranium ions uptake by natural zeolite.