A multicomponent space-charge transport model for an ion-exchange membrane composed of cylindrical pores with a variable radius and/or a variable wall charge density has been developed and tested for the Donnan dialysis separation of aqueous Cs+/Pb2+ and Na+/Pb2+ mixtures with a Nafion 117 cation-exchange membrane. Model equations for ion and water transport take into account ion/fixed-charge site electrostatic interactions, electric-field-induced water dipole orientation, ion-hydration free-energy changes during ion partitioning, concentration-dependent transport parameters, and ion-pair formation between absorbed Pb2+ and the membrane fixed charges. An excellent match of theory with experimental concentration vs time and water flow vs time data was achieved using pores with a constant radius and a repeating high/low square-wave surface charge distribution (where the low surface charge was a consequence of ion pairing). The model could not reproduce Donnan dialysis transport data when (1) the pore radius and wall charge density were constant and (2) the wall charge density was constant and the pore radius was a linear function of axial position. Pore radius changes had little effect on computed ion fluxes, but did produce large changes in the water flux across a cation-exchange membrane.