Osmotic coefficient data for aqueous sodium polyanetholesulfonic acid, sodium polyacrylate and polydiallyl dimethylammonium chloride solutions were determined at 298 K by employing the isopiestic method. The measured osmotic coefficients increase with increasing concentration in the experimental concentration range (0.1-1.5 m). A molecular thermodynamic model developed previously for polyelectrolyte solutions has been used to fit the experimental data. The concentration dependence of the osmotic coefficients can be described satisfactorily. 2002 Elsevier Science Ltd. All rights reserved.