The microstructure of the A1-Mn-Cr alloys (AI-5Mn-2.5Cr, A1-5Mn-4Cr and A1-5Mn-6Cr), which were produced by ultrasonic gas atomization and hot extrusion, has been characterized by a combination of differential scanning calorimetry, microhardness measurement, X-ray diffraction, and analytical transmission electron microscopy. The hardness of the as-solidified powder particles increased substantially when the chromium content increased, which was due to a decrease of supersaturation during rapid solidification because of extensive precipitation of 0-All~Cr 2. The hardness decreased with increasing powder particle sizes (i.e. cooling rates) for all three alloys since the amount of solid solution decreased. Ageing of the rapidly solidified AI-5Mn-2.5Cr powder particles resulted in the decomposition of the supersaturated microstructure and formation of precipitate phases. These were accompanied by hardness decreases and an increase in the aluminium lattice parameter. These reactions were the precipitation of the icosahedral I-phase at 210 °C, G-A112(Mn,Cr) at 300 °C and intergranular AI6Mn at 420 °C, respectively. In the higher chromium A1-5Mn-4Cr and A1-5Mn-6Cr alloys, only the precipitation of the A16Mn occurred at -436 °C. The higher strengths for the chromium-rich alloys were attributed to the larger volume fraction of precipitates. The best tensile ductilities were obtained for the A1-5Mn-2.5Cr alloy, in which the G-Al12(Mn,Cr) precipitated during extrusion.