<p>Understanding the phase equilibria of the CaO-Al₂O₃-La₂O₃-MgO system provides a crucial theoretical foundation for designing optimized refining slags used in rare earth steel production and for the modification of undesirable high-melting-point rare earth inclusions. An additional objective of this work was to explore the existence of any refractory quaternary compound in this system that could be suitable for development as an advanced thermal barrier coating (TBC) material. Experimental investigation of the phase relations for a selected composition region within the quaternary system was conducted at 1773&#xa0;K via the equilibration and rapid quenching technique. The equilibrium phases present were characterized and their compositions measured using scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), and x-ray diffraction (XRD). Based on these results, the 1773&#xa0;K isothermal tetrahedral phase diagram was developed, along with a series of corresponding pseudo-ternary sections at fixed w(MgO) levels. The established phase diagram features three four-phase fields (i.e. Liquid + 2CaO·3Al₂O₃·La₂O₃ + Al₂O₃·La₂O₃ + MgO, Liquid + 2CaO·3Al₂O₃·La₂O₃ + MgO·Al₂O₃ + MgO, Liquid + 2CaO·3Al₂O₃·La₂O₃ + CaO·2Al₂O₃ + MgO·Al₂O₃), 7 three-phase fields (i.e. Liquid + Al₂O₃·La₂O₃ + MgO, Liquid + 2CaO·3Al₂O₃·La₂O₃ + Al₂O₃·La₂O₃, Liquid + 2CaO·3Al₂O₃·La₂O₃ + MgO, Liquid + MgO·Al₂O₃ + MgO, Liquid + 2CaO·3Al₂O₃·La₂O₃ + MgO·Al₂O₃, Liquid + 2CaO·3Al₂O₃·La₂O₃ + CaO·Al₂O₃, Liquid + CaO·2Al₂O₃ + MgO·Al₂O₃), and four two-phase fields (i.e. Liquid + MgO, Liquid + 2CaO·3Al₂O₃·La₂O₃, Liquid + MgO·Al₂O₃, Liquid + CaO·Al₂O₃).</p> Graphical Abstract <p></p>