A paper presents results of studies of HTO transport in unsaturated soil in the vicinity of long-term emission source and tritium oxidation by soil microorganisms. The objective of the first study was to determine the vertical profile of tritium from surface to lOm-depth using boreholes. The boreholes were sampled every Im for measurements of tritium in the soil water. The objective of the second study was to determine tritium oxidation activity of soil in laboratory environment depending on temperature, tritium activity, humidity, etc. A special apparatus in which a mixture of air, water vapour and tritium gas is pumped through the soil material under study was used to determine soil oxidation activity. The amount of HT converted into HTO in the soil sample during a certain period of time was used to determine the tritium reaction rate This rate varies, depending on the catalytic and/or biological activity of the material. The knowledge of the reaction rale can help to estimate HT deposition rate of HT onto the surface of an ureusturbed soil sample. A theoretical treatment based on simplifying assumptions has shown that the deposition rate can be expressed by the reaction rate and the effective diffusion coefficient of HT in the porous material.The studies are performed in frame of ISTC project 654,

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