Reversal of the kyanite-sillimanite inversion has been accomplished hydrothermally at 750°C. The inversion pressure at 750°C is 8.1 ± 0.4 kilobars. The calculated pressuretemperature slope at this point is 17.7± 1.0 bars per degree celsius. Geologically, this result seems more plausible than previous estimates of the location of the boundary. When combined with other work on the relative stability of andalusite, the data indicate that andalulsite cannot be stable at a pressure greater than 4.2 kilobars.