Phase relations in the ternary system ZrO2-CaO-MgO were experimentally established at 1600degrees, 1700degrees, and 1750degreesC. The investigation was based on powder X-ray diffractometry, scanning electron microscopy-energy dispersive spectroscopy, and electron probe microanalysis, on 24 ternary compositions. The compositions were prepared using attrition milling of respective oxides and carbonates as raw materials. The results obtained allowed construction of the corresponding isothermal sections, which verified the existence of the cubic-ZrO2-CaZrO3 phase compatibility field at the three temperatures. Finally, experimental results also were compared with the thermodynamic assessment previously reported of the system ZrO2-CaO-MgO.