Shock deformation of Fe&sngbnd;Mn alloys up to 14 wt&percent; Mn results in a shock‐induced phase transformation. It has been shown that bcc martensite with manganese in the range 4–16 wt&percent; transforms to a stable close‐packed phase under pressure without the occurrence of reversion. The addition of manganese to iron also decreases the transition pressure from 133 kbar for pure Fe to less than 70 kbar for Fe&sngbnd;14Mn. X‐ray diffraction, electron‐probe microanalysis, electron microscopy, and density results indicate that for the Fe&sngbnd;4Mn and Fe&sngbnd;7Mn alloys, the fcc phase has been stabilized after shock deformation, while the &egr; phase has been stabilized for the Fe&sngbnd;14Mn alloy. Saturation magnetization studies have detected a residual reduction in magnetization due to the retainment of the high‐pressure phase.