Manganese is a grain refiner for high purity Mg–3%Al, Mg–6%Al, Mg–9%Al, and commercial AZ31 (Mg–3%Al–1%Zn) alloys when introduced in the form of an Al–60%Mn master alloy splatter but the use of pure Mn flakes and ALTAB™ Mn75 tablets shows no grain refinement. Long time holding of the melt at 730 °C leads to an increase in grain size. The mechanism is attributed to the presence of an ε-AlMn phase (hexagonal close-packed) in the master alloy splatter.