A membrane enhanced deionized capacitor (MEDC) device, consisting of insulating plates (6,7) and a series of MEDC unit cells, is provided. The MEDC unit cell includes a cation and anion ion-exchange membranes (2,4), one pair of carbon electrodes (1-1,1-2), a spacer (3) and an insulating holder (5) and is assembled in the order of [(1-1)/(2)/(3)/(5)/(4)/(1-2)]. The cation-exchange membrane combined with one electrode is used as negative electrosorptive electrode while the anion-exchange membrane combined with another electrode is used as positive electrode. Unit cells can be connected with each other in series, or in parallel. The devices can be made as either stack type or roll type.