This work is aimed at evaluating the affinity of partially ethoxylated polyethyleneimine (PEPEI) towards industrially valuable metal ions (Cu2+, Ni2+, Cd2+ and Zn2+). To face this aim, a characterization of polymer and macromolecular complexes by acid-base potentiometry was carried out. Next, lab-scale total recirculation mode ultrafiltration experiments were performed, checking that the affinity order was the same as that calculated from acid-base potentiometric titrations. Moreover, the influence of pH and loading ratio (LR) on Cu2+ and Zn2+ selective separation was evaluated, obtaining a maximum Zn/Cu selectivity of 12.31 at the optimal working conditions: pH 6, T=50 degrees C. Delta P=3 bar, LR=286.74 mmol Me2+/mol PEPEI. Finally, a model based on the competitive reaction between polymer functional groups and the solution cations was proposed. This model accurately adjusts metal rejection coefficients when pH is close to neutrality. Moreover, in order to overcome the drawbacks of acid-base potentiometry method, this model was used to calculate new complex formation constants from metal rejection data. (C) 2010 Elsevier B.V. All rights reserved.