Quantitative failure of the ideal Donnan model to predict ion partitioning between relatively dilute electrolyte aqueous solutions and ion exchange polymers arises from neglecting non-ideal behavior of ions. Hypothetically, when a water swollen, charged polymer is equilibrated with concentrated salt solutions, most of the fixed charge groups are neutralized by sorbed counter-ions, which can screen electrostatic effects and create, in the membrane, an environment thermodynamically similar to that experienced by ions in the external electrolyte solution. In this study, a combined experimental and theoretical approach was used to test this hypothesis. A fundamental study of ion partitioning between a cation exchange membrane based on cross-linked poly(p-styrene sulfonate-co-divinylbenzene) and NaCl and CaCl2 concentrated brines is presented. At high electrolyte concentrations, the experimentally measured ion activity coefficients in the membrane match those in the contiguous external solution, and the ideal Donnan model provides an accurate prediction of co-ion and counter-ion concentrations in the polymer. This physical picture was further confirmed by the recently developed Manning-Donnan model.