In order to reveal the reactivity of a functional group in an aromatic compound having two substituents inthe aromatic ring, the hydrogen-isotope exchange reaction (T-H exchange reaction) between tritiated water vapor(HTO vapor) and 4-amino-2-methylbenzenesulfonic acid (and 5-amino-2-methylphenol) were dynamically observedat 50 (and 70 ) in a gas ℃ ℃ -solid system. Consequently, the fact that the specific activity of the acid increased withtime was obtained, and the T-for-H exchange reaction occurred. By applying the A“-McKay plot method to the dataobserved, the rate constant of each functional group for the reaction was obtained. After the additive property of theHammett’s rule was applied to this work, the new substituent constants were obtained. From the above-mentioned,the following four items have been confirmed: (1) the reactivity of the functional groups can be dynamically analyzed,and the A”-McKay plot method is useful to analyze the reactivity; (2) the additive property of the Hammett’s rule isapplicable to quantitative comparison of the reactivity of the functional groups; (3) the reactivity of the functionalgroups can be simultaneously analyzed by using the A“-McKay plot method in the T-H exchange reaction; and (4) themethod used in this work is also useful for analyzing the reactivity of a certain material having some kinds of functional groups. In order to reveal the reactivity of a functional group in an aromatic compound having two substituents inthe aromatic ring, the hydrogen-isotope exchange reaction (TH exchange reaction) between tritiated water vapor (HTO vapor) and 4-amino-2-methylbenzenesulfonic acid and the fact that the specific activity of the acid increased with time was was taken, and the T-for-H exchange By reaction the A ”“ - McKay plot method to the dataobserved, the rate constant of each functional group for the reaction was obtained. After the additive property of the Hammett’s rule was applied to this work, the new substituent constants were obtained. From the above-mentioned, the following four items have been confirmed: (1) the reactivity of the functional groups can be predicted analyzed, and the A ”- McKay plot method useful to analyze the reactivity; (2) the additive propertyof the Hammett’s rule isapplicable to quantitative comparison of the reactivity of the functional groups; (3) the reactivity of the functional group can be simultaneously analyzed by using the A "- McKay plot method in the TH exchange reaction; and (4) themethod used in this work is also useful for analyzing the reactivity of a certain material having some kinds of functional groups.