The microstructure of an off-normal 12%Cr FV448-grade ferritic-martensitic weldment with 4.7 wt.% nickel addition has been compared with that of parent plate following fast reactor irradiation at 465°C to damage levels in the range 28–46 dpa. The fully martensitic weld metal exhibits instability under irradiation and transforms to a duplex austenitic/-martensitic structure with high void swelling in the austenite. Irradiation-induced precipitation differs between weldment and parent plate. M6X and G-phase are dominant phases formed in the plate; however, large δ-ferrite grains in the heat-affected zone contain only G-phase. Differences in precipitation in FV448 observed in earlier studies can be rationalized in terms of parameters which influence radiation-induced segregation.