Isobutene is the only product of the uncatalyzed and acid-catalyzed dehydration of tert-butanol in compressed liquid water at 250 degrees C. The uncatalyzed dehydration reaction is fast : equilibrium is established after about 30 s. Only one of many chemically-motivated kinetic models is able to fit all the experimental data. This model presumes a heterolytic dehydration mechanism that involves protonated alcohol, carbocation, di-tert-butyl ether, and protonated ether as intermediates. The model permits tert-butanol to dissociate as both an Arrhenius acid and a Bronsted acid while catalyzing its dehydration to isobutene. The pK(a) of tert-butanol at 250 degrees C is predicted to be about 9, whereas its value at normal temperature and pressure is 18. This 10(9) increase in the K-a value of tert-butanol hints of the promising new world of reaction chemistry in near- and supercritical water.