The relative stability of the assemblages clinozoisite-quartz and epidote-grossularite-anorthite have been determined between 1 and 4 kb using a rate technique in which the reaction is monitored with a single crystal of one of the participating phases. Clinozoisite and quartz react fractionally forming progressively more iron-rich epidote with anorthite and grossularite garnet as temperature rises. Between 1 and 7 kb, the relation T=533.6+34.5 P-0.6 P 2 (T degrees C, P kb) provides the best clinozoisite-quartz dehydration curve. Iron-rich epidote is stable with quartz at least 32 degrees above clinozoisite-quartz. Results indicate that where grandite and anorthite occur at the expense of clinozoisite and quartz in high grade Barrovian metamorphic rocks, temperatures must have exceeded 675 degrees C if P tot > or =6Kb,P water > or =1/2 P tot .