In the system CaO–MgO–Al2O3–SiO2, the equilibrium compositions of clinopyroxene and garnet coexisting with corundum were determined in 16 runs at 1300°C/19.5–31.6 kbar and 1500°C/29.8 kbar. The phase transition clinopyroxene → garnet (CaTs50Di46En4 → Gr64Py36) was reversed at 1100°C/25.0 kbar and 1300°C/32.5 kbar. In the Mg-free system, the breakdown curve of calcium Tschermak pyroxene to grossular and corundum was reversed at 1300, 1400 and 1500°C and can be described by the expression: P(bar) = 55T(°C) – 53900. The data were modeled using the Redlich-Kister equation, providing the mixing properties of the diopside–calcium Tschermak pyroxene and the grossular–pyrope solid solutions. The resulting model indicates complex short-range order–disorder phenomena in both solid solutions. Activities of the Ca-Tschermak component in pyroxene are substantially lower than the mole fractions. A discontinuity in the mixing properties of pyrope-rich garnets, caused by ordering of Ca and Mg in the dodecahedral site, is implied. The model predicts unmixing at 828°C/1 bar and 870°C/20 kbar, with the crest of the solvus at Gr70Py30. A major disagreement was observed between the excess enthalpies of Gr–Py garnets predicted by the model and those calculated from the published heats of solution measurements. Calculated phase relations of the assemblage clinopyroxene + garnet + corundum and a T–X phase diagram for the join grossular–pyrope at 30 kbar are presented.