Three zeolite-clay composite membranes have been used for separation of chloride salts of trivalent cations (FeCl3 and AICl(3)) in an unstirred batch cell. The membranes used are the unmodified analcime zeolite (Z(1)), the modified zeolite containing oxynitride groups (Z(2)) and the modified zeolite containing imine/amine groups (Z(3)). The experimental data has been analyzed by the irreversible thermodynamics approach and the concentration polarization is also taken into account in the model used. The calculations show that modification causes an increase in the intrinsic rejection (from similar to70% for the unmodified to similar to94% for the Z(2) and similar to96% for the Z(3) membrane in case of FeCl3 solution and from similar to84% for Z(1) to 90% for both Z(2) and Z(3) membranes in the case of AlCl3 solution) of the membrane and a decrease in the solute permeability. The reflection coefficients of the modified membranes are found to be more than that of the unmodified membrane and that of Z(3) membrane is found to increased to a value of about 1 (sigma = 0.978 for FeCl3 and sigma = 0.99 for AlCl3). (C) 2004 Published by Elsevier B.V.