A systematic investigation of the microstructures of a series of magnesium based die cast magnesium‐aluminium‐rare earth (RE) alloys has been performed using X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. The alloys had an aluminium content of 3‐4 wt-% and RE content of between 0·6 and 3·5 wt-%. The alloys were studied in the as cast condition and after aging at 200 and 250°C. Three kinds of binary aluminium‐RE phases were found in the alloys existing intergranularly. Al11RE3 was the predominant intergranular phase. In addition, Al3RE and Al2RE phases were found in the alloys with high RE content. The Al3RE phase became unstable when the alloy was aged at 200°C, while the Al11RE3 phase remained stable at 250°C. In alloys with a RE/aluminium weight ratio of <0·6, primary Al12Mg17 and aluminium enriched zones existed at the grain boundaries beside the Al11RE3 phase, both of which are considered to contribute to the reduction in creep resistance. The aluminium enriched zone transformed into Al12Mg17 precipitates when the alloys were aged. Therefore, to achieve improved creep resistance in Mg‐Al‐RE alloys, the RE/Al ratio should exceed 0·6.