Results are presented of a study of phase equilibria among crystalline and liquid phases in the quaternary system CaO–MgO‐Al2O3–SiO2at Al2O3contents greater than 35%. Equilibrium diagrams shown are for the five triangular joins CaAl2Si2O3‐Ca2Al2SiO7‐MgAl2O4, Ca2Al2SiO7‐MgAl2O4‐Al2O3, CaAl2Si2O8‐MgO‐Al2O3, CaAl2Si2O8‐Mg2SiO4‐MgAl2O4, and CaAl2Si2O8‐MgO‐Mg2SiO4. The composition and nature of the four quaternary peritectic points and the relationships of univariant lines and primary ph