An electrodeionization apparatus comprising: an anode compartment containing an anode and a cathode compartment containing a cathode an ion-concentrating compartment having opposed side surfaces at least partially bounded by an anion permeable membrane on one side surface on the cathode side and at least partially bounded by a cation permeable membrane on the other side surface on the anode side; and a first ion exchange material domain disposed within the ion-concentrating compartment, wherein the first ion exchange material domain is contiguous with at least a portion of an ion-concentrating compartment side surface (26, 28) of one of the anion permeable membrane and the cation permeable membrane, and is spaced apart from the other one of the one of the anion permeable membrane and the cation permeable membrane; wherein the at least a portion of the ion-concentrating compartment side surface of the one of the anion permeable membrane and the cation permeable membrane defines an operative side surface fraction of the ion-concentrating compartment side surface of the one of the anion permeable membrane and the cation permeable membrane; such that, in the case where the one of the anion permeable membrane and the cation permeable membrane, having the at least a portion of an ion-concentrating compartment side surface with which the first ion exchange material domain is contiguous, is an anion permeable membrane, the first ion exchange material domain is an anion exchange material predominant domain; and in the case where the one of the anion permeable membrane and the cation permeable membrane, having the at least a portion of an ion-concentrating compartment side surface with which the first ion exchange material domain is contiguous, is a cation permeable membrane, the first ion exchange material domain is a cation exchange material predominant domain; and characterized in that the electrodeionization apparatus further comprises a mixed ion exchange material domain disposed within the ion-concentrating compartment, wherein the disposition of the first ion exchange material domain relative to the other one of the one of the anion permeable membrane and the cation permeable membrane defines a respective space between the first ion exchange material domain and the other one of the one of the anion permeable membrane and the cation permeable membrane, and wherein at least a portion of the mixed ion exchange material domain is disposed within the respective space. an anode compartment containing an anode and a cathode compartment containing a cathode an ion-concentrating compartment having opposed side surfaces at least partially bounded by an anion permeable membrane on one side surface on the cathode side and at least partially bounded by a cation permeable membrane on the other side surface on the anode side; and a first ion exchange material domain disposed within the ion-concentrating compartment, wherein the first ion exchange material domain is contiguous with at least a portion of an ion-concentrating compartment side surface (26, 28) of one of the anion permeable membrane and the cation permeable membrane, and is spaced apart from the other one of the one of the anion permeable membrane and the cation permeable membrane; wherein the at least a portion of the ion-concentrating compartment side surface of the one of the anion permeable membrane and the cation permeable membrane defines an operative side surface fraction of the ion-concentrating compartment side surface of the one of the anion permeable membrane and the cation permeable membrane; such that, in the case where the one of the anion permeable membrane and the cation permeable membrane, having the at least a portion of an ion-concentrating compartment side surface with which the first ion exchange material domain is contiguous, is an anion permeable membrane, the first ion exchange material domain is an anion exchange material predominant domain; and in the case where the one of the anion permeable membrane and the cation permeable membrane, having the at least a portion of an ion-concentrating compartment side surface with which the first ion exchange material domain is contiguous, is a cation permeable membrane, the first ion exchange material domain is a cation exchange material predominant domain; and characterized in that the electrodeionization apparatus further comprises a mixed ion exchange material domain disposed within the ion-concentrating compartment, wherein the disposition of the first ion exchange material domain relative to the other one of the one of the anion permeable membrane and the cation permeable membrane defines a respective space between the first ion exchange material domain and the other one of the one of the anion permeable membrane and the cation permeable membrane, and wherein at least a portion of the mixed ion exchange material domain is disposed within the respective space.