We report, for the first time, the acceleration of acid-catalyzed reactions in high-temperature liquid water by the addition of carbon dioxide to the reaction medium. The reactions examined are the dehydration of cyclohexanol to form cyclohexene and the alkylation of p-cresol with tertbutyl alcohol to form 2-tert-butyl-4-methylphenol. In some cases, the mean product yield more than doubled when carbon dioxide was added to the aqueous medium. The basis of this rate enhancement is reaction between carbon dioxide and water to yield carbonic acid, which subsequently dissociates to increase the hydronium ion concentration above that of water alone. We also use the thermodynamics of the CO2-H2O system to estimate the pH of carbon dioxide-enriched high-temperature water. This analysis demonstrates that it is possible to lower the pH of high-temperature water by several units through the addition of carbon dioxide to the reaction medium. These results demonstrate the feasibility of using CO2-H2O mixtures as an environmentally benign reaction medium for acid catalysis.