The effects of the molecular weights and concentrations of three polymers, poly(acrylic acid sodium salt) (NaPA), polyacrylamide (PAA), and poly(ethylene glycol) (PEG), on the electrokinetic and rheological properties of sepiolite suspensions were investigated. The zeta-potential values of the sepiolite suspensions were measured at different pH values, and the isoelectric point of sepiolite was determined to be 3.27. The anionic NaPA polymers were much effective in reducing the zeta-potential values of sepiolite than the nonionic polymers PAA and PEG. PAA and PEG of higher molecular weights caused a significant increase in the zeta-potentials of the sepiolite suspensions as their concentrations and molecular weights increased. The suspensions exhibited Bingham flow behavior. The reason for the increase in the plastic viscosity values of the sepiolite/NaPA suspensions was the increase in the polymer concentration in the suspensions; on the other hand, for the sepiolite/PAA or PEG suspensions, the reasons were the increase in the polymer concentration in the suspensions and the sepiolite-polymer interactions. (C) 2008 Wiley Periodicals, Inc.