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.