The plot of log k(obsd) vs pH for the hydrolysis of o-carboxybenzaldehyde trans-1,2-cyclohexanediyl acetal at 50 degrees C in H2O has four unit changes of slope in the pH range 2-9. The plot is here described by proceeding from low pH to high pH. The observed hydronium ion- and water-catalyzed reactions at pH < 6 have rate constants that are similar, but not identical, to those for hydrolysis of the acylal 3-[(trans-2-hydroxycyclohexyl)oxy]phthalide, which was isolated from the reaction at FH 3. and synthesized independently. The pH-log rate constant profile for hydrolysis of the acetal bends downward near FH 6 to give a slope of -1.0. Oxocarbonium ion hydrolysis is then a water reaction. At pH 7 the mechanism of the reaction changes to attack of OH- on the oxocarbonium ion intermediate. A change in rate-determining step takes place at FH 8 to hydronium ion-catalyzed ring opening of the anionic species of the acetal, or the kinetically equivalent intramolecular general acid catalysis in ring opening of the neutral species. The mechanism involving general acid catalysis by the neighboring carboxyl group is strongly supported by the D2O solvent isotope effect. The o-carboxyl group enhances the rate of the acetal ring-opening reaction by a factor of 230 in comparison with the exactly analogous p-carboxyl-substituted acetal. in contrast, the analogous p-OCH3-, p-NO2-, o- and p-COOCH3-, and p-COOH-substituted derivatives have uncomplicated linear pH-log rate constant profiles with slopes of -1.0. A neighboring carboxyl group can participate in the hydrolysis of an acetal of an aliphatic alcohol if the C-O bond breaking process is facilitated by the release of steric strain. The implications of these results for the mechanism of lysozyme-catalyzed reactions are discussed.