To answer the question of whether the anionic tetrahedral intermediates produced from base hydrolysis of esters are in protonic equilibrium, O-18 exchange studies of the esters ethyl toluoate (2) and isopropyl toluoate (3) recovered from the basic hydrolysis medium in H2O and D2O at 25 degrees C were undertaken. The observed deuterium solvent kinetic isotope effects on (k(hyd))(H2O/D2O) are 0.75 and 0.81 for 2 and 3, respectively. For the O-18=C exchange process, the DSKIEs are (k(ex))(H2O/D2O) = 0.97 and 0.86 for 2 and 3. Overall (k(ex)/k(hyd))(H2O) = 0.068, (k(ex)/k(hyd))(D2O) = 0.052 for 2; for 3 these values are (k(ex)/k(hyd))(H2O) = 0.15, (k(ex)/k(hyd))(D2O) = 0.14 On the basis of these results there is no compelling evidence for a process limiting the exchange where there is a proton in flight generating a strong primary deuterium kinetic isotope effect.