The hydrolysis of hydrochlorothiazide and two other hydrothiazides was studied as a function of pH. Reversible kinetics were observed for the hydrolytic reaction, and a bell-shaped pH-rate profile was obtained. The equilibrium constant, however, was relatively independent of hydrogen-ion concentration. The hydrolytic rate constants and the equilibrium constants for the overall reaction were evaluated. General catalysis was checked at several pH values, utilizing acetate and imidazole buffers, and no significant buffer catalysis was observed. The bell-shaped pH-rate profiles cannot be explained completely by the ionization of reactants, but they can be interpreted by postulating the presence of an imine intermediate that undergoes attack by water or hydroxide to form a carbinolamine which subsequently decomposes to products.