The stability field of anorthite was studied from 950 to 1640 degrees C and 6 to 37 kb. The following is known: (a) 3 An<-->grossularite + 2 kyanite + quartz, and (b) 3 An<--> 3 Tschermak molecules + 3 quartz <--> grossularite + 2 kyanite + quartz. The equilibrium boundary of (b) occurs at higher temperatures than (a). The boundary of the An breakdown to grossularite + 2 kyanite + quartz lies at a lower pressure, but rather close to that of albite. An melts incongruously at pressures above 9 kb to corundum + liquid.