In perfluorocarboxylate ion exchange membrane-aqueous sodium chloride system, the diffusional flux of sodium ions against their own concentration difference was observed in the presence of a pH difference across the membrane. The internal solution contained 1 x 10(-1) mol dm(-3) NaOH and 1 x 10(-1) mol dm(-3) NaCl, and the external solution contained 2 x 10(-1) mol dm(-3) NaCl and HCl of various concentrations in the range of 1 x 10(-2) to 1 x 10(-1) mol dm-3. In these membrane systems, it was observed that the membrane potential rapidly changed in response to a pH jump in the external side of two aqueous phases to reach an intermediate stage and then the subsequent step started to relax slowly to the final membrane potential at the other steady state. On the basis of the assignment that the earlier fast step and subsequent slow step observed in the generation process of the membrane potential are the generation processes of the Donnan potential at the membrane/solution interface and the intramembrane diffusion potential, respectively, the total membrane potential has been divided into these two constituents. By using the observed Donnan potential, the ion concentration at the membrane surface in the membrane was obtained. The ion flux was analyzed to obtain the diffusion coefficient of ions within the membrane by using the ion concentration at the membrane surface in the membrane and the intramembrane diffusion potential. The pH dependence of the diffusion coefficient and that of the ion concentration at the surface in the membrane showed a break point near the apparent pK(a), where the transition of the ion cluster structure in the membrane would occur.