AbstractFe–Cr alloys and the ferritic phase of duplex stainless steels may undergo embrittlement during aging in the temperature range 300–500°C, owing to a spinodal reaction process. The atom probe at the University of Oxford. has been used to investigate the microstructural changes accompanying the spinodal in a low carbon, low molybdenum steel (ASME SA351 CF3) in the temperature range 300–400°C for aging times of up to 20 000 h. The amplitude and wavelength of the spinodal were determined. In addition, some other steels, which in previous studies by Trautwein and Gysel had exhibited an extremely low value of activation energy for embrittlement, were examined using both the atom probe and the position sensitive atom probe. Some of this material was re-heat treated to attempt to explain its anomalous behaviour. Steel from the primary cooling circuit of the Shippingport reactor was also examined and carbon and phosphorus were found to have segregated to a phase boundary.MST/1202