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The hydrogen isotope exchange reaction between HTO vapor and each amino acid has been observed in order to establish a method of estimation of the internal exposure of organically bound tritium in a heterogeneous system at 25∼70°C. Rate constants (k) for the amino acids have been obtained by applying the A″-McKay plot method. Using these k values, Arrhenius plots for both the COOH and NH2 groups are drawn, and linearity was obtained over the range of 25∼70°C. Comparing the rate constants, the following four statements can be made regarding the T-for-H exchange reaction. (1) The reactivity of the functional groups in amino acids increases with increasing temperature. (2) Applying Taft's equation, the ratio of polar effect to steric effect is 2:8 in the COOH group and 9:1 in the NH2 group at 25°C. (3) The A″-McKay plot method is useful for studying the reactivity of materials, not only with one (or the same kind of) functional group(s) but also with two different kinds of functional groups. (4) The method u...