The isoplethal sections CaAl2O4-MgO and CaAl4O7-MgO of the Al2O3-MgO-CaO ternary system have been experimentally established at 1 bar total pressure and air of normal humidity. The sections obtained provide new data and information that are in disagreement with thermodynamic evaluations and optimizations of the Al2O3-MgO-CaO ternary system published to date. These differences arise mainly from the inclusion, or exclusion, of the binary compound Ca12Al14O33, mayenite, as a stable phase in the reported studies of the system. The presence or absence of this compound within the system has an important impact on the solid state and melting relationships of the whole ternary system. The present study confirms the solid-state compatibility CaAl2O4-MgO and CaAl2O4-MgO-MgAl2O4 up to 1372 degrees +/- 2 degrees C, the peritectic melting point of the later mentioned subsystem.