In situ synchrotron diffraction experiments were performed during the solidification of a Mg4Y3Nd alloy. The material was melted and solidified inside a sealed stainless steel crucible in the chamber of a Bähr 805 A/D dilatometer. The sample was heated up to 680°C and kept at this temperature for 5 min to ensure it is molten. Afterwards it was cooled down to the fully solidified state with a cooling rate of 10 K/min. During the T(t) program diffraction patterns were acquired continuously in every 25 s (∼5 K). The forming phases were identified as α-Mg at 625°C, Mg₁₂Nd and Mg₁₄Y₄Nd at 545°C, and Mg₂₄Y₅ at 320°C. The experimental results were correlated with simulations based on thermodynamic databases.