Gas medium and solid medium pressure apparatus were used. The lower temperature experiments were done hydrothermally. Only great changes, about 20 percent conversion of one polymorph to another, were accepted as diagnostic. The preferred equation expressing equilibrium results is P(kb)=0.0243T( degrees C)-9.63. Maximum uncertainties in metamorphic ranges are included. The presence of water has no effect on the position of the transition, although it speeds attainment of equilibrium. Geological applications of the results to the problems of load pressure in metamorphism can be made in areas where the temperature of the kyanite-sillimanite isograd is determined independently.