It is found that the spontaneous magnetization of disordered iron‐aluminum alloys changes the mode of theL20order‐disorder reaction from a second to a first degree transition. A new two‐phase field (&agr;m+ &agr;2n) has been found in the composition range 22.3 at. % Al to 25 at. % Al and this is responsible for many of the magnetic anomalies of these alloys. The microstructure of alloys heat‐treated in the (&agr;m+ &agr;2n)‐phase region can be controlled by an externally applied magnetic field which can also be used to determine quantitatively the ``antiphase'' boundary energy of theL20structure.