The present paper deals with the electrokinetic characterization of sepiolite. A series of systematic zeta potential measurements have been carried out to determine the isoelectric point (iep) and potential-determining ions (pdi), and the effect of mono-, di, and trivalent electrolytes such as NaCl, KCl, LiCl, NaNO3, NaCH3COO, MgCl2, CaCl2, BaCl2, CoCl2, CuCl2, Pb(NO3)(2), Na2CO3. Na2SO4, AlCl3, FeCl3, and Na3PO4 on the zeta potential of sepiolite. Zeta potential has been calculated with the aid of Smoluchowski's equation. Sepiolite yields an isoelectric point at pH 6.6. The zeta potential for the sepiolite has ranged from +23.3 mV at pH similar to 2 to -22.4 mV at pH similar to 8 at 20 +/- 2 degreesC in water. The valency of the ions have proven to have a great influence on the electrokinetic behavior of the suspension. Monovalent cations were found to have a weak effect, while di- and trivallent cations made the zeta potential positive. Charge reversal was observed for divalent cations at 1 x 10(-2) M and for trivalent cations at 3 x 10(-4) M. As a result, it can be said that monovalent cations are indifferent ions when di- and trivalent cations are potential-determining ions. (C) 2004 Elsevier Inc. All rights reserved.