Activity expressions for Mg3Al2Si3O12 and Ca3Al2Si3O12 in pyrope-grossular garnets that include explicit enthalpy, entropy, and volume terms have been formulated from measured thermodynamic properties. Predicited Ca3Al2Si3O12 activities agree well with those given by Hensen et al. (1975) based on high-pressure phase equilibrium experimentation, when the latter activities are corrected to 1 bar with the measured partial molar volumes. Calculations of the stabilities of garnets in the CaO-MgO-Al2O3-SiO2 system reproduce fairly accurately the most reliable phase equilibrium data at 1000¿-1200¿C that do not heavily involve a clinopyroxene phase. Though the calculation of garnet-clinopyroxene stability relations is in general agreement with available experimental observations, a critical comparison is hindered by the uncertainties in the high-temperature heat capacities of the garnet and clinopyroxene components. The predicted solvus in the garnet join develops a 'spike' at high pressures which is displaced toward pyrope-rich compositions.