The Mg rich corner of the ternary Mg–Si–Sr alloy system is experimentally and thermodynamically investigated in this work. Thermodynamic simulation of the ternary phase diagram required modelling of the ternary phases that occur in these alloys, namely the ternary intermetallics MgSiSr and MgSi₂Sr for which only scarce information is available in literature. The Mg-rich side of the Mg–Si–Sr phase diagram is constructed based on descriptions of the binary phase diagrams Mg–Si, Mg–Ca and Ca–Si from literature and assuming complete solubility (i.e. a line compound) between the ternary phase MgSiSr and the binary phase Sr₂Si. It is also assumed that MgSi₂Sr is a stoichiometric compound. A good agreement is found between the constructed phase diagram and the experimental microstructural results for a range of cast and heat treated alloys.