The reaction of muscovite plus quartz to sanidine plus mullite was investigated to six kilobars H 2 O pressure. The results of lowpressure runs agreed well with previous work. The reaction of muscovite plus quartz to muscovite plus liquid was investigated to 20 kilobars. The pressure coefficient of melting of a synthetic quaternary granite, where muscovite is the first phase to appear on the liquidus, is 6.5 degrees C per kilobar.