Mg-RE alloys have a good potential to be used in bio-applications as degradable implants. Their macroscopic characteristics are strongly dependant on the microstructure, which can be tailored through the alloy composition and the solidification parameters. In situ synchrotron diffraction provides a unique tool to follow the phase formation, possible meta-stable and stable phase transformations and grain growth during cooling. In the present study Mg alloys containing Gd, Y and Nd were investigated to characterize the solidification phenomenon during cooling from 680°C to room temperature. Samples were melted and solidified in a Bähr 805 dilatometer modified for in situ synchrotron measurements. The molten samples were contained in steel crucibles, and the temperature was controlled by type S thermocouples during cooling. The results provide an experimental validation of thermodynamic calculations; and provide experimental input for refining the thermodynamic models, which contribute to the better understanding of the microstructure evolution to control desirable macroscopic characteristics.