The process of spontaneous transport of an ion from one aqueous solution (W1) to another (W2) through a liquid membrane (LM) coupled with the electron transfer was elucidated based on the ion or electron transfer voltammograms measured between W1 and W2 and at the W1/LM or LM/W2 interfaces. Based on the above fundamental research, the membrane separation of K+ from Na+ with the aid of electron transfer was discussed.