The rheological and colloidal properties of sodium montmorillonite dispersions were investigated in the presence of a special type of cationic polymer [modified poly(ethylene glycol)]. 2-(Dimethylamino) ethyl methacrylate was polymerized with monomethoxy-capped oligo(ethylene glycol) via aqueous atom transfer radical polymerization. The tertiary amine residues of the resulting polymer were then quaternized with methyl iodide to obtain a cationic polyelectrolyte. The rheology and zeta-potential experiments showed that the cationic polymer adsorbed onto the sodium montmorillonite surface strongly. The rheological parameters (plastic viscosity and yield value) were obtained with a rotational low-shear rheometer. The results indicated a gradual increase in gelation with the addition of the cat-ionic polymer, which reached a maximum at a cationic polymer concentration of 0.4-0.8 g/L. This gel-like dispersion showed pronounced thixotropy. A further increase in the polymer concentration resulted in a reduction in this gelation. The adsorption of the cationic polymer onto the clay surface reduced the 6 potential to small values, but no isoelectric point was observed. The basal-spacing measurements showed that the cationic polymer strongly adsorbed onto the sodium montmorillonite instead of entering the montmorillonite layers. (C) 2004 Wiley Periodicals, Inc.