Radiation-induced isotopic equilibration in hydrogen gas containing tritium has to be accounted for when cryogenic distillation of hydrogen isotopes is used for their separation. In the absence of sufficient experimental data on the reaction kinetics an appropriate theoretical model is proposed. Unknown parameters of the model have been estimated with use of Möller-Plesset ab initio method. The results of calculations are in a reasonable agreement with certain experiments performed earlier by other authors. The dependencies of equilibration rate on temperature, pressure, and tritium molar fraction are established for isotope mixtures containing no impurities.