A device for the electrodeionisation of an aqueous electrolyte solution, comprising an electrodeionisation module (1), an ultrafiltration module (21, 22, 1201, 1202, 1203) and a connection piece (3, 10) which connects the electrodeionisation module (1) and the ultrafiltration module (21, 22, 1201, 1202, 1203) such as to be able to guide a diluate produced from said electrolyte solution during operation of said device in said electrodeionisation module (1) from said electrodeionisation module (1) to said ultrafiltration module (21, 22, 1201, 1202, 1203); wherein said connection piece (3, 10) comprises no adjustable pressure-maintaining valve. By means of the pressure drop occurring in the ultrafiltration modules (21, 22, 1201, 1202, 1203) these devices build up a counter pressure behind the electrodeionisation module (1) which is sufficient to ensure the packing density of the ion exchanger in the electrodeionisation module (1).