Estazolam deuterated in the triazole ring was isolated and its carbon-13 nuclear magnetic resonance signals were assigned in relation to those of estazolam. The coupling constant ratio, [numerical formula], for the triazole C-1 atom was 6.66. Rates of hydrogen exchange of estazolam with deuterium were measured at 23°C in acidic aqueous solution (0.5 N DCl) and in methanol-d4 contaning trifluoroacetic acid-d and triethylamine. The exchange reaction was accelerated in the presence of acid or base, indicating that the exchange occurs in a base-catalyzed reaction and that the triazole C-1 atom of estazolam behaves as an acid.