The kinetics of the UV-stimulated HT oxidation in oxygen atmosphere was studied in combination with computer simulation consisting of 30 elementary reactions. The rate of HTO formation was observed to be proportional to the pressure of O2 and molar fraction of HT with respect to total hydrogen pressure (H2 + HT), and 1/2 order to total hydrogen pressure. The simulation resulted in the same pressure dependence as the observed one. The contributions of elementary reactions to the HTO formation were calculated, assuming steady states for intermediate species. It was revealed that the UV-stimulated HT oxidation is initiated by photolysis of O2 to O(3P) (λ < 242 nm) and subsequently formed O3 and OH play important roles in the HTO formation. The present results suggest that the conversion of tritium gas to tritiated water is considerably enhanced in the stratosphere owing to the presence of UV photons having shorter wavelengths than 242 nm.