Measurements of bi-ionic potential (BIP) across three cation-exchange membranes (CEMs) have been carried out for KCl/CEM/NaCl, KCl/CEM/LiCl, and NaCl/CEM/LiCl systems, where CEM is a polystyrene and divinylbenzene sulfonated membrane (CM2, from Tokuyama Soda), a perfluorosulfonic acid membrane (Nafion(R) 117, from Du Pont De Nemours) and an heterogenous membrane prepared by inclusion of cation-exchange resin in PVC (CRP, from Phone Poulenc). The influence of the salt concentration and the diffusion boundary layers (DBLs) on the BIP values has been analysed both theoretically and experimentally. The theoretical model is based on the Nernst-Planck equations, and gives a good description for salt concentrations higher than 5 x 10(-4) M. For the CM2 membrane, the DBL thickness changes from 20-23 mu m in absence of stirring to 3-4 mu m for high stirring rates. Also, the ion diffusion coefficients in this membrane have been estimated to be of the order of 10(-6) cm(2)/s. It has been observed that the counter-ion diffusion coefficients ratio (<(D)over bar (A)>/<(D)over bar (B)>) in the membrane increases significantly when the membrane water content decreases, which suggests the possibility of achieving highly selective ion transport with low water content membranes.