A key phase diagram study of the Li₂O-MgO and Li₂O-CaO systems at 1 atm was performed using equilibration/quenching experiments and differential scanning calorimetry measurements with sealed 90Pt-10Rh crucibles. In particular, the eutectic temperatures of both systems and the solubility of Li₂O in solid MgO and CaO were properly measured for the first time. The eutectic temperatures of the Li₂O-MgO and Li₂O-CaO systems were determined to be 1696 ± 6 K (1423 ± 6 °C) and 1617 ± 6 K (1344 ± 6 °C), respectively. A solubility of 2.6 mol% (1.9 wt%) Li₂O in solid MgO was measured at 1743 K (1470 °C) and no detectable solubility of Li₂O in solid CaO was determined. The present experimental results were employed for the thermodynamic modeling of the binary Li₂O-MgO and Li₂O-CaO systems. The phase diagrams for the binary Li₂O-MgO and Li₂O-CaO systems were constructed for the first time, and the phase diagram of the ternary Li₂O-MgO-CaO system was predicted from the binary descriptions.