A series of model steels and commercial alloys from the IAEA Phase 3 irradiation programme was irradiated to a dose of 14 mdpa (∼ 9 x 10 22 n m -2 (E > MeV)) at 290°C. The steels were examined using small angle neutron scattering and a range of transmission electron microscope techniques. Measured yield stress and hardness changes were interpreted in terms of the irradiation-induced microstructural developments. The irradiation of Cu-containing steels produced Cu-rich precipitates of ∼ 2 nm diameter which were alloyed with Mn and Ni. The Ni content of the precipitates increased markedly for bulk Ni contents > 0.7 wt%. The volume fractions of precipitate increased with increasing bulk Cu content and, also, with increasing bulk Ni content > 0.7 wt%. The precipitate-induced strengthening and hardening varied according to the square root of the volume fraction of precipitate. Yield stress changes in low-Cu steels were consistent with data from the IAEA Phase 2 irradiation programme.