Methemoglobin can be reduced on a platinum cathode using flavin mononucleotide as an oxido-reduction mediator. The process requires the utilization of a filter-press cell with compartments separated by a semi-permeable membrane. Analysis of the various constraints imposed by the process itself and by the nature of the molecules involved shows that the electrolysis cell must operate at a low temperature, in strictly anaerobic conditions, in series with a storage tank, and with fluid circulation rates lower than approximately 0.8 m/s. A process has been designed that takes into account these imperatives and enables volumes of solution of the order of 200 cm(3) to be processed. It enables optimization of the flow rates used as well as of the methemoglobin/flavin ratio and is the forerunner of an industrial reactor.