The equations that relate the dissociation constants between divalent metal ions and polymer chains in aqueous solutions with the ionic strength are developed from a model previously discussed-that gives account for the experimental results obtained in ultrafiltration experiments. The relations found are used to predict the retention values at high filtration factors in ultrafiltration experiments corresponding to the systems poly(sodium 4-styrenesulfonate)/Cd2+, and poly(vinylalcohol)/Pb2+ respectively, performed under variable initial ionic strength.