The present invention relates to a process for the deionization of a cooling medium in a fuel cell (11) circulating in a cooling circuit (20), in which the cooling medium is subjected to at least intermittent, but preferably continuous, electrochemical deionization. To this end, at least one electrode deionization cell (23), through which a diluate stream (27) serving as cooling medium and a concentrate stream (28) flow, is arranged in the cooling circuit. The concentrate stream (28) may be part of a secondary cooling circuit.