An osmotic pressure-driven model for uptake of water in rubber is combined with dual-sorption-site theory to describe the transport of water in highly plasticized, poly-(vinyl chloride) (PVC)-based ion-selective electrode membranes, both at equilibrium and during the initial stages of water uptake. During the first hour, mobile, miscible water is predicted to have a diffusion coefficient given by D mo ) D/(1 + C i s /s o ), where D mo and D are the observed and true diffusion coefficients, respectively, C i s is the molar concentration of salt impurity and s o is the maximum miscible water concentration. Addition of CoCl 2 to PVC plasticized with dioctyl adipate (DOA) or nitrophenyl octyl ether (NPOE) showed that D mo followed this expression during initial stages of water uptake. Values of D ) (1.5 ( 0.1) × 10 -6 cm 2 /s and s o ) 51 ( 5 mM, versus D ) (4.3 ( 0.2) × 10 -7 cm 2 /s and s o ) 82 ( 7 mM, were obtained for membranes with 66% DOA or NPOE as plasticizer, respectively. Added CoCl 2 or KB(C 6 H 5 ) 4 reduced D mo . The dependence of equilibrium water content on added salt and on ionic strength is also predicted by the theory, and qualitative agreement with theory was observed experimentally. Addition of 0.15% CoCl 2 to a PVC/DOA membrane increased the equilibrium water content from 0.36 ( 0.05 to 3.04 ( 0.05 M. Light scattering by water droplets formed in the membrane was decreased by increasing the ionic strength of the contacting aqueous solution. For an NH 4 + -selective membrane used as an optode, this effect introduced absorbance changes as a function of ionic strength.