Liquidus temperatures and phase equilibria have been determined in the olivine primary phase field of the MgO-“FeO”-SiO₂-Al₂O₃ system. Liquidus isotherms have been determined in the temperature range from 1748 to 1873K. The results are presented in the form of pseudo-ternary sections of the MgO-“FeO”-SiO₂ with 2 and 3 wt% Al₂O₃ in the liquid. The study enables the liquidus to be described for a range of SiO₂/MgO ratios. It was found that liquidus temperatures in the olivine primary phase field decrease with the addition of Al₂O₃.