Die-cast MRI230D and AXJ530 have superior creep resistance to MRI153M, despite their similar microstructure, primary eutectic phases and dislocation substructure after creep. In order to better understand this difference in creep resistance, solute concentrations in the primary α-Mg in as-cast Mg-Al-Ca-based alloys have been examined using the electron microprobe along with a. Scheil analysis. The amount of Al in solution in the primary α-Mg is of particular interest since it can significantly affect creep behavior, due to precipitation and/or solid-solution strengthening. MRI230D and MRI153M both have higher levels of Al in the primary alpha, but only MRI230D also has increased levels of Ca as well, which may be attributable to the presence of Sn in this alloy. The differences in the primary alpha compositions may explain the differences in creep behavior that are observed.