The sorption of U(VI) from aqueous solutions on zeolite X has been studied by a batch technique. Distribution coefficients (K-d) were determined for sorption systems as a function of sorbate concentration, pH, contact time and temperature. The sorption isotherm was formed according to the Langmuir isotherm. Thermodynamic parameters have been determined at different temperatures. The Delta H degrees values for U(VI) on zeolite X were -29.5147 kT mol(-1) at 313 K at pH 3 and -19.8705 kJ mol(-1) at 303 K at pH 9. The sorption of U(VI) on zeolite X is an exothermic in nature. Negative values of Delta G degrees show the spontaneous values for U(VI) that become less negative at higher temperatures, which shows that sorption is less favoured at higher temperatures. (C) 1998 Elsevier Science S.A.