Phase equilibria and liquidus temperatures in the CaO–SiO₂–Al₂O₃–MgO system with CaO/SiO₂ weight ratio of 1.10 have been experimentally determined by means of high temperature equilibration and quenching technique followed by electron probe X-ray microanalysis. Isotherms in the interval of 20 K between 1613 and 1733 K were determined in the primary phase fields of dicalcium silicate, wollastonite, merwinite, periclase, spinel and melilite that are relevant to ironmaking slags. Effects of Al₂O₃, MgO and CaO/SiO₂ ratio on the liquidus temperatures have been discussed to assist optimum operation of iron blast furnace. Compositions of the solid solutions corresponding to the liquidus have been accurately measured that will be used for development of the thermodynamic database.