Laboratory experiments and field observations have shown that elemental tritium HT is completely converted into tritiated water ( HTO ) by the activity of soil microorganisms. No organically bound tritium ( OBT ) is formed initially, but some tritium is taken up into soil biomass by general biosynthesis. Only a small fraction of tritium is directly incorporated from HTO into OBT with the non-exchangable portion becoming the dominant source of tritium in OBT. In practice a small fraction of non-exchangeable OBT must be separated by vacuum distillation from a large amount of soil water containing HTO. The method has been tested by labelling the soil water with HTO and H218O, respectively. Several steps are necessary to obtain a complete yield. During the incubation experiments a continuous loss of HTO by an exchange between soil water and air water vapour must be taken into account. Uptake of tritium into biomass of about 0.1 % per week was observed. The biological synthesis and consequently the uptake of tritium from HTO into OBT can be stimulated by the addition of energy sources such as glucose.