Within the framework of the European fusion technology programme, SCK/CEN has started the development of a 100 ml per day electrolyser for decomposing the highly tritiated water that will be formed when tritium is extracted from the breeder or when the plasma exhaust is purified. Safety and reliability of this electrolytic system will have to be guaranteed for at least 104 working hours. Three different cell configurations are being studied one of which is most promising because of its low tritiated water inventory (∼ 12 ml), its low working temperature (< 10 °C) and other advantages such as avoiding any recirculation of radioactive streams.