A device for the electrodeionization of an aqueous electrolyte solution, comprises an electrodeionization module (1), an ultrafiltration module (21, 22, 1201, 1202, 1203) and a connection piece (3, 10) which connects the electrodeionization module (1) and the ultrafiltration module (21, 22, 1201, 1202, 1203) such as to be able to guide a diluate produced from the electrolyte solution during operation of the device in the electrodeionization module (1) from the electrodeionization module (1) to the ultrafiltration module (21, 22, 1201, 1202, 1203); wherein the connection piece (3, 10) is devoid of an adjustable pressure-maintaining valve. Via the pressure drop occurring in the ultrafiltration modules (21, 22, 1201, 1202, 1203), these devices build up a counter pressure behind the electrodeionization module (1) which is sufficient to ensure the packing density of the ion exchanger in the electrodeionization module (1).