From the analysis of the membrane potential, the magnitude of the Donnan potential was calculated by using the values of the measured parameters. The Donnan potential is the electric potential at the inner position located just a little off the interface and mainly reflects the electric charge within the membrane. On the other hand, the zeta (zeta) potential is the potential at the slipping plane. The slipping plane is at the outer position just a little off the interface. Therefore, it is possible to obtain more precise information on the distribution of the membrane charge, especially at the interface, by comparing the Donnan potential with the zeta-potential. According to this concept, we compared the Donnan potential with the zeta-potential measured for the nylon 6 membrane at various pH values. As a result, it is pointed out that the nylon 6 membrane dissociates only around the membrane/bulk solution interface but scarcely within the membrane. Within the membrane, the effective membrane charge is determined by selectively adsorbed ions. This conclusion suggests that the electrochemical properties of membranes are different from those proposed based on the properties of membrane materials in aqueous solution.