A molecular-level equilibrium partition coefficient model has been formulated and tested for the uptake of aqueous monovalent/divalent cation-salt mixtures into a Nafion 117 perfluorosulfonic acid cation-exchange membrane. The model utilizes a simple cylindrical-pore microstructure for Nafion and considers ion hydration free energy changes which occur during solute partitioning, the orientation of water dipoles within a membrane pore due to the strong electric field generated by the membrane’s fixed-charge sites, and the formation of contact ion pairs (coordinate covalent bonds) between divalent cations and membrane ion-exchange sites. Membrane structure parameters in the model (pore-radius and pore-wall fixed-charge concentration) were determined from membrane porosity experiments and X-ray diffraction data in the literature. When the fraction of sulfonate sites which bind to divalent cations was used as an adjustable parameter, the model was able to predict accurately the divalent/monovalent cation selectivity (membrane-phase concentrations) of Pb2+ and Cd2+ With a coabsorbed alkali metal cation (either Li+, Na+, K+, or Cs+) for external salt concentrations of 0.05 and 0.25 M. Independent verification and quantification of the extent of divalent cation binding were made by NMR analyses of salt-equilibrated membranes. The NMR results and model predictions for the fraction of total membrane-phase lead that was bound to sulfonate sites were found to be in excellent agreement with an average error less than 6% for a series of 16 uptake experiments with Pb2+ and either Li+ or K+.