The effect of pressure on the aluminum—manganese binary system was investigated by preparing various Al‐Mn alloys in a cubic multi‐anvil pressure apparatus. Pressures of 40 000 and 60 000 atm were used in preparing the alloys from powdered Al and Mn mixes. Indirect heating was used to melt the specimens and to apply a variety of heat treatments (including a magnetic anneal) to the alloys while they were under pressure. The x‐ray results indicate that pressures of 40 000 atm have little effect on the Al‐Mn system, since the phases prepared under pressure were the same as those found at 1 atm by other investigators of this system. A new identification of the phases present in the &eegr;, &eegr;′ region was made. One phase is body‐centered cubic (a0=8.9 Å), the other is face‐centered cubic (a0=12.5 Å). It was not possible to ascertain to which phase, &eegr; or &eegr;′, the abovea0values belonged. The results obtained at a pressure of 60 000 atm differed from those found at 40 000 atm as follows: (a) when quenching alloys in the 50–60 at. % Mn region from 1550°C at 60 000 atm, the magnetic &tgr; phase is formed in preference to the &egr; phase which is formed at 40 000 atm, and (b) a new phase, the structure of which is being determined, was formed from the &eegr;,&eegr;′ phases. Several specimen magnets were made by the high‐pressure technique; their magnetic properties were comparable to as‐cast AlMn magnets before they were given optimizing treatments.