The adsorption of the cationic polymer, polyethyleneimine (PEI), on bentonite clay particles was investigated. The adsorption rates for the bentonite suspensions are very fast. The high affinity type of interactions between PEI and bentonite samples is attributed to the electrostatic interaction between the clay particles in ambient basic suspensions and the positively charged high molecular weight polyelectrolyte, The adsorption capacity and the influence of the polyelectrolyte on the rheological behavior change with the exchangeable ions, i.e. Na and Ca ions of the bentonite samples. (C) 2002 Elsevier Science B.V All rights reserved.