[en] Microstructural examinations are reported for nine specimen conditions of 2-1/4Cr-1Mo steel which had been irradiated by fast neutrons over the temperature range 390 to 510⁰C. Two heats of material were involved, each with a different preirradiation heat treatment, one irradiated to a peak fluence of 5.1 x 10²² n/cm² (E > 0.1 MeV) or 24 dpa and the other to 2.4 x 10²³ n/cm² (E > 0.1 MeV) or 116 dpa. Void swelling is found following irradiation at 400⁰C in both conditions and to 480⁰C in the higher fluence conditions. Concurrently dislocation structure and precipitation formed. Peak void swelling, void density, dislocation density and precipitate number density developed at the lowest temperature, approx. 400⁰C, whereas mean void size, and mean precipitate size increased with increasing irradiation temperature. The examination results are used to provide interpretation of in-reactor creep, density change and post irradiation tensile behavior