The determination of the electrokinetic potential of clay is very important in many applications. Therefore, this work was aimed at studying the effect of poly(4-sodium styrene sulfonate) on the electrokinetic potential and the sedimentation behavior of kaolinite clays at different pH values and/or in the presence of sodium or calcium chlorides. At pH values lower than the point of zero charge (PZC) of the kaolinite clay, the zeta potential of Na- or Ca-kaolinite in CaCl2 solutions was higher than that in NaCl solutions at the same concentration. Above the PZC, the zeta potential of Na- or Ca-kaolinite in NaCl Solution Was higher than that in CaCl2 at the same electrolyte concentration. These results reflect the ability of calcium cations to compress the double layer of the kaolinite clay better than sodium cations. Also, below the PZC and at low concentrations of poly(4-sodium styrene sulfonate), the zeta potential of the kaolinite clay Suspensions had low value, while the sediment Volumes had higher values. On the other hand, above the PZC at increasing polymer concentrations, the zeta potentials increased but the sediment volumes had lower values compared to those produced below the PZC. The data showed the zeta potential of Na-kaolinite suspensions are higher than Ca-kaolinite, producing lower sediment volumes of Na-kaolinite compared to Ca-kaolinite at the same concentrations of polymer and/or pHs. (c) 2006 Wiley Periodicals, Inc.