Previous investigations show that CaO additions have an effect on the microstructure and mechanical properties of commercial Mg alloys. Addition of CaO to pure Mg and Mg alloys results in the dispersion of Ca in the molten and solid Mg with no evidence of CaO. This paper reports on the reactions between Mg, Al and CaO measured by means of in situ synchrotron radiation diffraction during melting and solidification. The experiment was conducted on Mg-3A1 wt% with CaO concentrations between 3 wt% and 10 wt% to study the effect of different Al:CaO ratios. The initial sample contained Mg, Al and CaO in powder state, pressed to cylindrical samples. The melting, the solidification and associated phase transformations were measured in a Bahr 805A/D dilatometer inside a sealed stainless steel crucible at the High Energy Materials Science Beamline P07 at PETRA III, DESY.