In order to reveal the reactivity of a functional group in an aromatic compound having two substituents in the aromatic ring, the hydrogen-isotope exchange reaction (T-for-H exchange reaction) between tritiated water vapor (HTO vapor) and 3-amino-4-methoxybenzenesulfonic acid (and 2-aminotoluene-5-sulfonic acid) was dynamically observed at 50°C (and 70°C) in a gas-solid system. Consequently, the specific activity of the acid increased with time, and it showed that the T-for-H exchange reaction occurred. Applying the A″-McKay plot method to the data observed, 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 dynamically analyzed, and the A″-McKay plot method is useful to analyze the reactivity. (2) The additive property of the Hammett's rule is applicable to quantitative comparison of the reactivity of the functional groups. (3) The reactivity of the functional groups can be simultaneously analyzed by using the A″-McKay plot method in the T-for-H exchange reaction. (4) The method used in this work is also useful for analyzing the reactivity of a certain material having some kinds of functional groups.