The determination of the electrokinetic properties of colloidal systems is very important for the chracterization of these systems. Colloidal systems have high adsorption performance due to the carrying of negative charges and the colloid structure. The control of the electrokinetic properties of the bentonite-water system are important not only from a technological point view; they are also important from a scientific point of view. Knowing the electrokinetic and rheological properties of bentonite minerals is important for the estimation of the behavior of clays under various environmental conditions. The purpose of this study was to interpret the effect of the nonionic poly(ethylene glycol) (PEG) polymer on electrokinetic and rheological properties. Zeta potential and viscosity measurements were done as a function of PEG molecular weights (400, 3000, and 8000) and their concentrations (2.5 X 10(-5) to 1.25 X 10(-2) mol/L. We interpreted the experimental data, taking into account these two parameters. X-ray diffaction studies were done together with the electrokinetic and rheological measurements. (C) 2002 Wiley Periodicals, Inc.