The removal of cadmium ions from aqueous solutions where chitosan was present was carried out by polymer enhanced ultrafiltration (PEUF). The effect of pH and polymer/metal ratio was studied. Essentially, chitosan binds cadmium ions at neutral pH, but the protonation of amine groups leads to the release of Cd2+ when pH is lowered. A two-phase model, previously developed for a soluble ion-exchange applied to water softening, was adapted for complexation reactions. This model considers the feed solution to be divided into two regions. One region is occupied by the polymer, the bounded ions and some water; the other region is the remaining solution. The estimation of two parameters, the equilibrium constants corresponding to the formation of the complexes Cd(R-NH2)(2+) and Cd(R-NH2)(2)(2+), allows to calculate cadmium rejections that agree with the experimental ones. The advantage of a two-phase model versus a single-phase one is discussed. (C) 2004 Elsevier B.V. All rights reserved.