The interactions of water-soluble polymers with metal ions are treated as surface phenomena in which different metal species compete with each other to bind to the polymer surface. The adsorption isotherms for the system poly(sodium 4-styrenesulfonte)/Cd2+/Na+ are obtained by ultrafiltration of the polymer at constant ionic strength in the presence of 0.15, 0.10, 0.05, 0.02, and 0.01 M NaNO3 solutions, respectively. A change in the rate of release of divalent metal ions by the polymer is detected which is attributable to a change in the polymer conformation. A Michaelis-Menten-like mathematical development is shown for the competition of both metal ions.