The process of the self-sustained oscillation of the membrane potential was elucidated referring to voltammograms for ion transfer at two interfaces existing in the membrane system. The membrane system investigated was composed of one aqueous solution, W1, containing cetyltrimethylammonium chloride, CITA(+)Cl(-), a nitrobenzene liquid membrane (LM), containing picric acid and another aqueous solution, W2, in the absence of any salt. The oscillation of the membrane potential was attributed to that of the potential difference at the W2 \ LM interface, E-W2 \ LM, and E-W2 \ LM was demonstrated to oscillate between that determined mainly by the transfer of H+ and Pic(-) from LM to W2, and that determined by the transfer of H+ and Cl- from LM to W2. Fundamental features such as the membrane potential during the induction time and the amplitude of the oscillation predicted based on the process proposed in the present work agreed quantitatively with those observed with the membrane system.