A complete literature review, critical evaluation and thermodynamic modeling of phase diagrams and thermodynamic properties of all oxide phases in the CaO–MgO–SiO2 system at 1 bar pressure are presented. The molten oxide phase is described by the Modified Quasichemical Model, and the Gibbs energies of the olivine and pyroxene solid solutions are modeled using the compound energy formalism. A set of optimized model parameters of all phases is obtained which reproduces all available and reliable thermodynamic and phase-equilibrium data within experimental error limits from 25 °C to above the liquidus temperatures over the entire composition range. The complex phase relationships in the system have been elucidated, and discrepancies among the data have been resolved. The database of the model parameters can be used along with software for Gibbs energy minimization in order to calculate all thermodynamic properties and any phase diagram section or phase equilibrium of interest.