Abstract: Studies have been made on muscovite single crystals treated under dry and hydrothermal conditions at temperatures above 800°C. The course of reaction is markedly affected by the amount of water present in the system. In the presence of a large excess of water, appreciable reaction commences at 800°C with the formation of kalsilite and corundum. The kalsilite may bear an epitaxial relationship to the muscovite, but the corundum is quite unoriented. Reaction appears to proceed by solution of part of the muscovite and reprecipitation on unreacted areas. In the presence of low pressures of water the reaction course appears to be similar to that under dry conditions. Above 900°C a new phase has been identified which is similar to the spinel phase reported by Eberhart at temperatures above 1050°C. The phase is monoclinic and probably represents an intermediate stage between dehydrated muscovite and the spinel phase. A tentative mechanism is proposed for the dehydration of muscovite based on an inhomogeneous mechanism.