In this contribution, the material accountancy principle is applied to a tritium laboratory subsystem. Although, for the time being, we expect only relatively small amounts of tritium to be handled in units in the fuel cycle the methodology to be used has to cover a substantial increase of tritium operation and consequently in the amount of tritium; it seems mandatory that the tools used are able to cover all aspects, especially those of complete fuel cycles, for example NET. The statistical elements are known from other implementations in the nuclear field; that knowledge is presented here for the first time in the very special framework of tritium accountancy.