The materials under study are 9–12% Cr ferritic/martensitic steels, alternative candidate materials for application in core components of nuclear power reactors. This work involves: (1) investigation of high temperature fracture mechanism during slow tensile and limited creep testing at 600°C; (2) extensive study of solute element segregation, theoretically using proposed model of combined equilibrium and non-equilibrium mechanisms; and experimentally using field emission gun scanning transmission electron microscopy; (3) investigation of effects by thermal ageing and irradiation on microstructural developments in relation to high temperature mechanical behaviour. Quenching rates, tempering temperature and tensile test temperature variations are included in the thermomechanical treatments.