Mg-RE alloys have potential to be used for bio-applications as degradable implant materials. For the production of these alloys casting plays an important role: Phase formation and microstructure evolution are at first determined during solidification. In situ synchrotron radiation diffraction is a unique tool to follow phase formation, phase transformations and grain growth during solidification. The measurements were performed at the High Energy Materials Science beamline P07 at PETRA III at the Deutsches Elektronen-Synchrotron (DESY, Hamburg, Germany). In the present study Mg4Y, Mg4YlGd and Mg4Y4Gd alloys were investigated to characterize their solidification behavior during cooling from 800°C and 680°C to 450°C, respectively with a cooling rate of 20 K/min. The samples were held in graphite crucibles and were molten in a modified Bahr 805 dilatometer under Ar atmosphere, while the temperature was controlled by type S thermocouples. The results are compared with thermodynamic simulations using Pandat 8™ software.