A study has been made of both the transformation behaviour and mechanical properties of a series of very low carbon iron manganese alloys within the range 0 to 10% manganese. Alloys containing between 4 and 10% manganese were found to undergo a martensitic transformation which was identical to the lath or massive martensite reaction found in other iron alloys. A possible bainitic reaction was also-found in alloys containing between 4 and 6% manganese, and was referred to as recovery assisted martensite owing to the possible assistance to growth by recovery processes in either the austenite or martensite. In the lath martensitic condition the iron manganese alloys were shown to possess good tensile strengths and ductilities but their impact properties were markedly inferior to equivalent iron nickel alloys. An explanation of this poor impact resistance was sought in terms of both structural peculiarities, i.e. twinned martensite formation and from any deviation between iron manganese and iron nickel alloys in their dependence of flow stress on temperature and strain rate, neither of these two avenues of investigation revealed any source of explanation, and the most likely cause of brittleness was shown to be the possible segregation of impurities at prior austenite grain boundaries which created all of the effects usually encountered with temper brittleness.