Knowledge of the relevant stable equilibrium phase diagram is a prerequisite for taking account of deviations from equilibrium when modeling the solidification of silicon cast irons. While a linear description is practical for silicon contents up to 3 wt%, the curvature of the austenite liquidus at higher silicon contents necessitates the use of second-order polynomials. This study was carried out with the aim of obtaining an accurate description up to 4.5 wt% silicon, representative of today’s emerging high-silicon cast irons. In addition, alloying with up to 1 wt% copper, 0.5 wt% manganese and 0.25 wt% chromium and phosphorus was considered. In parallel to a description of the liquidus of austenite and graphite, the austenite-liquid partition coefficients of all alloying elements have been described. This paves the way for future work aimed at providing an accurate description of microsegregation and other non-equilibrium phenomena occurring during the solidification of silicon cast irons.