The phase dissolution and evolution during the melting and the solidification of Mg containing 10 wt.% CaO was investigated in the temperature range of 20-680 °C. The dissolution of CaO and the formation of Mg₂Ca were detected with in situ synchrotron radiation diffraction. The dissolution of CaO was observed at ∼407 °C with the detection of a peak unique to the Mg₂Ca phase prior to melting of Mg. After the solidification no CaO was detected, and Mg₂Ca and MgO phases were observed.