Alprazolam underwent a facile 1,4-benzodiazepine ring-opening reaction in an acidic aqueous solution to form a benzophenone compound. The reaction was demonstrated by means of UV, IR, and 1H- and 13C-NMR spectroscopy. Its reverse cyclization reaction to alprazolam occurred when an acidic solution was neutralized. Both the ring-opening and the cyclization rate constants were obtained from the overall rate constant measured at 25 degrees over a pH range of 0.5--8.0; the latter was measured by monitoring the UV spectral change of the reaction. Although the equilibrium was favored for the benzophenone compound in acidic solutions, it was possible to directly measure the cyclization rate at three acidic pH values by providing a sink condition for the product, alprazolam, using a biphasic reaction system. The bell-shaped cyclization rate pH profile was interpreted in terms of a change in the rate-determining step. The pH profile of the ring-opening rate showed an inflection point indicating a different reactivity of mono- and dicationic alprazolam. The apparent equilibrium between alprazolam and the benzophenone compound at a given pH was estimated from the rate constants for the ring-opening and cyclization reactions. The results agree with the apparent pKa measured by a conventional UV spectrophotometry and a titration technique. The pKa of monocationic alprazolam, the reactive species for the covalent hydration, was determined from the pH dependence of the initial absorbance when an alprazolam solution is acidified.