The creep behavior of cast Mg-Al-Ca-based alloys depends strongly on precipitation and solid solution strengthening in the primary a-Mg cells. In MRI230D, the presence of Sn may influence the elemental segregation during solidification, which in turn can affect precipitation and solid solution strengthening. The influence of Sn on elemental partitioning during solidification has been systematically investigated for the quaternary Mg-Al-Ca-Sn alloy system using an electron microprobe technique coupled with a Scheil analysis. Pandaf™ thermodynamic software has been used to predict the phases present and approximate the microsegregation in these alloys. The addition of 0.75-lwt% of Sn to Mg-5Al-3Ca causes an increase in the amount of Al and Ca in the primary a-Mg phase as compared to AX44 or AXJ530, although the increase is not as dramatic as the one observed in MRI230D. Sn concentrations of greater than lwt% lead to an increase in the Ca2-xMgxSn phase and a subsequent decrease in the amount of Ca in the a-Mg phase.