New data concerning the exsolution of plagioclases appear to modify the equilibrium curve between plagioclase and epidote. Eskola (1914) and J. H. L. Vogt (1927) contended that if sufficient water is present for the formation of epidote, plagioclase and epidote are unstable together. T. F. W. Barth (1928 and 1936), T. Strand (1942), I. Rosenqvist (1942) and J. A. W. Bugge (1943) have discussed the equilibrium between plagioclase and epidote, and H. Ramberg (1944) proposed a tentative diagram for the stability relations between these two minerals. Ramberg's diagram was based on empirical data, and because of experimental difficulties the system has not been experimentally examined.