The objective of this study was the examination of the ability of a natural zeolitic tuff to remove hexavalent uranium from aqueous solutions under various conditions. In order to achieve this goal, batch experiments were conducted to investigate the effect of pH, retention time, solid/liquid ratio and the presence of CO32- and SO42- ions in the solution. The results indicated that the natural zeolitic tuff is an effective material in the pH region 5-9 when the content of carbonates in the solution is controlled by the dissolution of the atmospheric carbon dioxide. In the case that the content of carbonates in the solution is controlled by geochemical processes and thus is enough high, the natural zeolitic tuff was proven an ineffective material. The interpretation of the results based on the aqueous chemistry of U(VI) containing aqueous solutions as well as on the physicochemical properties of the natural zeolite/ solution interface indicates that the U(VI) uptake by the zeolitic tuff is attributed to an adsorption mechanism. (C) 2003 Elsevier Ltd. All rights reserved.