This paper first presents a set of experimental data on the measurements of the ionic fluxes through a cation-exchange membrane separating two electrolyte solutions with the same concentration, the same co-ion but different counter-ions (bi-ionic system). Then these results are compared with the theoretical predictions given by the resolution of the corrected Nemst-Planck's equation in the cases of the homogeneous and heterogeneous models of ion-exchange membranes. Good agreement between the experimental results and the computed values is observed only in the case of the heterogeneous model. This confrontation confirms (1) the existence of three concentration domains: low, medium and high concentrations, corresponding, respectively, to a complete diffusion boundary layers (DBLs) control, a mixed control and a complete membrane control of the inter-diffusion process; (2) the homogeneous model, even if it is convenient for a good interpretation of the bi-ionic potential (BIP) results, cannot explain quantitatively the transmembrane ionic fluxes, which are of extensive quantities; and (3) the bi-phasic model gives the best predictions for both fluxes and BIPs.