A complete mathematical model that describes the metal ion retention profiles in ultrafiltration experiments of solutions of high molecular weight polymers and metal ions is given on the basis of polymer-metal ion interactions. The model allows the calculation of the free metal ion fraction and the fraction of metal ions bound to the polymer, as well as the equilibrium constants. Interactions between polyelectrolytes and metal ions are usually described by Manning＇s counterion condensation theory. The experimental results for the retention of Cd2+ in the presence of Na+ by the polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS) are in agreement with the predictions derived from Manning＇s counterion condensation two-variable theory.