In order to reveal the reactivity of aliphatic dicarboxylic acids, the hydrogen-isotope exchange reaction (T-for-H exchange reaction) between poly (vinyl alcohol) labeled with tritium at hydroxy group and one of aliphatic dicarboxylic acids in 1, 4-dioxane has been observed in a liquid-solid system in the range from 30 to 90°C. It was consequently found that (1) the greater the number of methylene groups in a aliphatic dicarboxylic acid is, the smaller is the reactivity of the acid; (2) a large difference is found between the reactivity of acids having even numbers of methylene groups and those having odd numbers, and the reactivity of the former is generally larger than that of the latter; (3) relating to the decrease of the reactivity of the acids with increasing number of methylene groups, the rate of the decrease in the case of even numbers is smaller than in the case of odd ones; (4) the reactivity of aliphatic dicarboxylic acids in T-for-H exchange reaction is largely dependent on the configuration (e.g., cisoid or transoid) of the two COOH groups.