This paper reports the adsorption of a cationic polyelectrolyte polyethyleneimine (PEI) onto a processed bentonite clay. The adsorption, rheological behaviours, and electrophoretic mobility of the clay suspensions were determined as a function of the addition of PEI concentration. Adsorption isotherm was obtained using the batch equilibrium technique and showed a high-affinity L-type behaviour. The experimental data points were fitted to the Langmuir equation with 0.9984 correlation coefficient. The saturation adsorption value was 0.3 g PEI/g clay. The rheological behaviour of the clay suspensions was obtained by shear stress-shear rate measurements within 0-350s(-1) shear rates. The sample follows the Bingham plastic flow model. Flocculation of suspension occurs at 10 % of maximum coverage of the clay particles and continues to approximately 50 % of surface coverage. Both the adsorption capacity and the shear stress values of the processed clay were several magnitudes higher than those of the natural clay. High-affinity type of interactions between PEI and bentonite samples shows a strong electrostatic interaction between the clay particles and the positively charged high-molecular-weight polyelectrolyte. X-ray diffractometry was used to investigate the intercalation behaviors of polymer-clay composites. (C) 2004 Elsevier B.V. All rights reserved.