The development of alloys with reduced long-life radioactivity has been incorporated into the goals of fusion materials development. One possible candidate alloy is an Fe-12Cr-6.5Mn-lW-0.3V-0.1C stabilized martensitic steel. The phase stability of this steel has been investigated following neutron irradiation over a temperature range of 420 to 600° C to doses as high as 100 dpa. The microstructural response of the irradiated steel has been compared to that of the same alloy aged for 10000 h from 365 to 600°C. M23C6 was the only carbide found in both the aged and irradiated structures. In the samples aged at 420 and 520°C, and the samples irradiated at 520°C, an Fe-Cr-Mn-W chi-phase formed. In the samples irradiated at 420°C, alpha prime formed. The presence of chi-phase in both the thermally aged and neutron irradiated steels indicates that the Fe-Cr-Mn system is prone to intermetallic phase formation so that careful study of the effects of alloying are needed. © 1991.