The dephosphorylation of p-nitrophenyl phosphate by Yersinia protein-tyrosine phosphatase (PTPase) and by the rat PTP1 has been examined by measurement of heavy-atom isotope effects at the nonbridge oxygen atoms [18(V/K)nonbridge], at the bridging oxygen atom [18(V/K)bridge], and the nitrogen atom in the leaving group 15(V/K). The effects were measured using an isotope ratio mass spectrometer by the competitive method and thus are effects on V/K. The results for the Yersinia PTPase and rat PTP1, respectively, are 1.0142 +/- 0.0004 and 1.0152 +/- 0.0006 for 18(V/K)bridge; 0.9981 +/- 0.0015 and 0.9998 +/- 0.0013 for 18(V/K)nonbridge; and 1.0001 +/- 0.0002 and 0.9999 +/- 0.0003 for 15(V/K). The magnitudes of the isotope effects are similar to the intrinsic values measured in solution, indicating that the chemical step is rate-limiting for V/K. The transition state for phosphorylation of the enzyme is dissociative in character, as is the case in solution. Binding of the substrate is rapid and reversible, as is the binding-induced conformational change which brings the catalytic general acid into the active site. Cleavage of the P-O bond and proton transfer from the general acid Asp to the leaving group are both far advanced in the transition state, and there is no development of negative charge on the departing leaving group. Experiments with several general acid mutants give values for 18(V/K)bridge of around 1.0280, 15(V/K) of about 1.002, and 18(V/K)nonbridge effects of from 1.0007 to 1.0022. These data indicate a dissociative transition state with the leaving group departing as the nitrophenolate anion but suggest more nucleophilic participation than in the solution reaction.