In this study, electrokinetic properties of clinoptilolite have been determined. A series of systematic zeta potential measurements have been performed to determine the isoelectric point (iep) and potential determining ions (pdi), and the effect of mono- and multivalent salts such as NH4Cl, CaCl2, Al(NO3)(3) on the zeta potential of clinoptilolite. Clinoptilolite in water exhibits a negative surface charge and maintains this character even in very acidic conditions of pH 2-3. This reveals that clinoptilolite has no iep in the pH range of 2-12. H+ and OH- ions are the pdi for clinoptilolite. Excluding the trivalent Al3+ cations, monovalent and divalent cations are not able to reverse the surface sign of clinoptilolite. This is attributed to the ion exchange ability of clinoptilolite and the strong specific adsorption of the Al3+ counterions. A schematic adsorption model which accounts for the adsorption of mono- and multivalent ions in the electrical double layer of clinoptilolite is proposed. (C) 2002 Elsevier Science Inc. All rights reserved.