The hydrolysis of the ortho ester 1,1-diethoxy-1,3-dihydroisobenzofuran (1) is described. This hydrolysis proceeds with initial expulsion of an exocyclic ethoxy group, producing upon hydration of the oxocarbonium ion 1-hydroxy-1-ethoxy-1,3-dihydroisobenzofuran (7), the tetrahedral intermediate of the lactonization of ethyl (2-hydroxymethyl)benzoate (6) to phthalide (8). Based on the kinetic behavior of the ortho ester hydrolysis, the pH variation in the ratio of6:8initially formed from the ortho ester, and the kinetic behavior of the lactonization reaction, rate constants for the breakdown of the tetrahedral intermediate7have been determined, along with the equilibrium constant for its formation from the hydroxy ester6. The breakdown occurs in an extremely rapid hydroxide-ion-catalyzed reaction, with the rate being at the diffusion limit. Hydronium ion catalysis is also observed; this breakdown is also rapid although considerably less so than the hydroxide ion reaction. The changeover from H+ to OH−catalysis occurs at pH 3.5. The rate of breakdown is a minimum at this point, and corresponds to a half-life for the tetrahedral intermediate of about 0.1 s. It is concluded that this allows sufficient time for conformational changes in the tetrahedral intermediate prior to breakdown; this factor must be taken into account in stereoelectronic considerations.