Thermochemical and thermophysical data for phases in the system MgO-SiO2 are critically evaluated, refined, and supplemented by calculating phase equilibrium relations from the available calorimetric data. Lattice vibrational models are used to constrain further the entropies and heat capacities. Phases in the data base are MgO, SiO2 (coesite, stishovite), Mg2SiO4 (olivine, β-phase, spinel), and MgSiO3 (pyroxene, garnet, ilmenite, and perovskite). The internally consistent data set reproduces the experimentally determined phase relations in the system at pressures to 30 GPa and to melting temperatures. ¿ American Geophyscial Union 1990