This work investigates the effect of AlZr master alloys, in-situ and synthetic produced particles, on their grain refinement behaviour within Mg-Al alloys. The effect of the master alloys with different solute and particle contents, on the grain size was measured. The in-situ particles were formed by reacting conventional master alloys. Samples were taken in accordance with the TP-1 test procedure (Aluminum association) and the resulting grain size of the Mg measured using the intersect method. A DSC and an SEM equipped with EDS, were employed to elucidate the effect of the Zr addition and synthetic particles. Results show for all test series a decrease in the grain size, with an ultimate grain size below 100 m for the synthetic particles being reached. Al3Zr phases were only detected when using the AlZr-alloyed master alloys. Furthermore, a poisoning effect concerning the efficiency of the synthetic particles within a Mn alloyed Mg-Al alloy was observed. In these series of tests the particles became ineffective. In conclusion Mg-Al alloys can be successfully grain refined using heterogeneous particles while these particles do not negatively affect the mechanical properties of the alloy.