Experiments were conducted to measure the isothermal rate of methyl tert-butyl ether (MTBE) decomposition in water at hydrothermal conditions (T = 150-600 degreesC and P = 250 bar). The primary identified products of the reaction of MTBE were methanol, isobutene, and tert-butyl alcohol. An assumed first-order rate constant for the hydrolysis of MTBE was determined at each temperature and showed a local maximum below the critical temperature of water (374 degreesC) followed by a local minimum above it. This behavior was modeled using an acid-catalyzed mechanism, which resulted in a rate expression with a first-order dependence on the concentration of H+. Because of the dramatic decrease in the ion dissociation constant of water (K-w) in the critical region, the drop in the rate of reaction could be modeled quantitatively over the entire temperature range.