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A new thermodynamic evaluation of the well-known Mg-Si system is presented with the aim to resolve persistent uncertainties in the Gibbs energy of its only compound, Mg2Si. For this purpose the heat capacity and enthalpy of melting of Mg2Si were measured by differential scanning calorimetry. Using finite temperature density functional theory and the quasiharmonic approximation, thermodynamic properties of Mg2Si were additionally calculated up to and above its melting temperature. Using these new data, in particular the heat capacity, the Mg-Si system was evaluated thermodynamically with the CALPHAD method leading to a thermodynamic description of the system within narrow bounds. In contrast to several previous evaluations there is no problem with an inverted miscibility gap in the liquid. Although present enthalpy of melting data turned out to be inconsistent with other data in this system, the new evaluation accurately describes all other available data in this system. In particular the Gibbs energy of Mg2Si can now be considered reliably described.