In order to quantitatively evaluate the internal exposure of tritium(3H or T), both the reactivity of exchangeable and non‐exchangeable Organically Bound Tritium(OBT) must be observed at body temperature. In this study, we assumed the model of exchangeable OBT, and the hydrogen isotope exchange reaction(T-for-H exchange reaction) between L-tyrosine and HTO vapor was observed at 25 and 36 ℃ in the gas-solid system. Applying the A″-McKay plot method to the data obtained in the reaction, the rate constants(k) of the functional groups in the material were obtained and Arrhenius plots were also drawn. In addition, from the Arrhenius plots in the range of 50 - 70 ℃, the estimated rate constants(kex) of 36 and 25 ℃ were obtained by using extrapolation method. Comparing k and kex, following three matters have been found in the T-for-H exchange reaction. (1)Because the value of k was similar to kex, extrapolation method is useful. (2)Applying Taft's equation, polar and steric substituent constants of C7H7O were obtained. (3)The method used in this work is useful for quantitative clarification of the reactivity of the T-for-H exchange reaction, and for the estimation of the internal exposure of exchangeable OBT.