There has been considerable interest in Al-rich Al-Mn-Ce alloys due to the variety of crystalline and quasi-crystalline metastable phases that can be formed. Here we report a study of the effects of heat treatment on an Al-5Mn-2Ce (at.%) alloy processed by gas atomization and consolidated by warm extrusion. Characterization using X-ray diffraction and electron microscopy showed that the powder microstructure consists mainly of an amorphous phase, FCC Al, and a previously unreported phase, Al20Mn2Ce. The extrudate is fully devitrified and contains a mixture of FCC Al, Al20Mn2Ce, and Al6Mn, with a small amount of Al12Mn and Al11Ce3. Upon heat-treatment at up to 450 A degrees C, the Al20Mn2Ce and Al6Mn phases decompose to give a hard stable phase mixture with 72-73 % Al12Mn plus 13-14 % each of Al11Ce3 and FCC Al. Heat treatments at 500 A degrees C give a much softer phase mixture consisting of 60 % FCC Al, 22 % of an unknown Al-3(Mn,Ce) phase, 9 % Al12Mn, 8 % Al6Mn, and 1 % Al11Ce3. The formation of large volume fractions of Al12Mn for heat-treatments at up to 450 A degrees C suggests that the presence of Ce may stabilize this phase, and that more dilute Al-Mn-Ce compositions could form the basis for new high-strength, low-density Al-based alloys with enhanced elevated temperature properties.