The reaction mechanism for Cu[9]aneN(3)Cl(2)-catalyzed hydrolysis of ethyl it-nitrophenyl phosphate was probed using kinetic isotope effects and isotope exchange experiments. The solvent deuterium isotope effect ((D)k = 1.14), combined with the absence of O-18 incorporation into 4-nitrophenol, suggests that hydrolysis proceeds through intramolecular attack of the metal-coordinated hydroxide at the phosphorus center. The secondary N-15 isotope effect ((15)k = 1.0013 +/- 0.0002) implies that loss of the leaving group occurs at the rate-limiting step with approximately 50% bond cleavage in the transition state. This study is one of the first applications of the secondary N-15 isotope effect to simple metal-promoted hydrolysis reactions, and the result is consistent with concerted bond formation and cleavage. A mechanism consistent with the isotope studies is presented.