The microstructure of the low activation F82H ferritic/martensitic steel has been investigated in the case of the heat treated and irradiated material. The irradiation has been achieved with 590 MeV protons in the PIREX facility to a dose of 0.5 dpa at a temperature of 523 K. The mechanical properties being intrinsically related to the chemistry of the metal, the understanding of the segregation behavior of the different alloying elements to microstructure defects, such as boundaries, is crucial. The segregation behavior to irradiation of both martensite laths and pre-austenite grain (PAG) boundaries is investigated using energy filtered transmission electron microscopy (EFTEM). It appears that Fe depletion of boundaries is observed in all cases, and the heat treated material experiences Cr segregation to boundaries, while the irradiated material experiences Cr depletion. (C) 1998 Elsevier Science B.V. All rights reserved.