In situ synchrotron radiation diffraction experiments were performed during the solidification of Mgl5Gd at the P07 (HEMS) Beamline of PETRA III at DESY. The measurements were carried out in the chamber of a modified DIL 805A/D dilatometer with a beam energy of 100 keV. The temperature was controlled by type S thermocouples welded on the steel lid of the graphite crucibles containing the samples. The two dimensional diffraction patterns were recorded with a Perkin Elmer 1621 Flatpanel. The phase evolution observed during cooling at rates of 5, 20 and 100 K/min show formation of GdMg3 at elevated temperatures, which transforms into GdMg5 during continued cooling. Phases were identified with the information from the Pearson's database for crystalline structures. This is different from that predicted with thermodynamic databases. Although the equilibrium phase diagram suggests a simple eutectic solidification, the experiments show a metastable phase formation and its transformation. The formation of GdMg3 becomes more pronounced at higher cooling rates.