All available thermodynamic and phase diagram data of the Mg–Si and Mg–Sn binary systems, and the Mg–Si–Sn ternary system have been critically evaluated and all reliable data have been simultaneously optimized to obtain one set of model parameters for the Gibbs energies of the liquid and all solid phases as functions of composition and temperature. The liquid phase was modeled using the Modified Quasichemical Model in order to describe the strong ordering in Mg–Si and Mg–Sn liquids. The Mg2Si–Mg2Sn solid solution phase was modeled with consideration of the solid miscibility gap. All calculations were performed using the FactSage thermochemical software.