For the reaction 3CaAl2Si2O8 = Ca3Al2Si3O12 + 2Al2SiO5 + SiO2, the logarithm of the apparent distribution coefficient (KD) is log KD = 3XgarnetCa3Al2Si3O12 − 3XplagloclaseCaAl2Si2O8. Log KD varies from —0.5 to —1.4 for kyanite-bearing assemblages and from —2.1 to —3.4 for sillimanite and/or andalusite-bearing assemblages. Using equilibrium constant equations derived from experimental data on pure phases and an ideal solution model for garnet and plagioclase solid solutions, individual P-T curves can be calculated for different values of log KD. If P or T can be independently estimated, these curves can be used as either geothermometers or geobarometers. The degree of non-ideality of grossular solid solution in garnet can be estimated from calculations using the activity coefficients of anorthite in plagioclase and the kyanite-sillimanite P-T equilibrium curve as a limiting case for kyanite and sillimanite-bearing assemblages.