A kinetic study on the base-catalyzed acyl group-exchange reactions of N,O-diacyl-o-aminophenols was undertaken to show that the formation of the amidate ion should be the rate-determining step in these intramolecular acyl-exchange reactions, and that the electron-withdrawing power of acyl group becomes an important factor to control the relative stabilities of a pair of N,O-diacyl-o-aminophenols.