An Fe-1200ppmP-30ppmC alloy has been examined after aging and irradiation exposure at 240°C. Thermal aging for 97 days reduced the yield stress by 10% but increased the 40J Charpy transition temperature by 50°C. Irradiation for a similar duration, in a highly thermalised neutron spectrum at 5.8 ± 1.7×10-11dpa/sec, increased both the yield stress by 15% and the transition temperature by 70°C. Longer irradiation, in spectra with or without the high thermal fraction, produced additional hardening but little extra embrittlement. Irradiation and aging gave a fivefold increase in the fraction of brittle intergranular fracture. Auger Spectroscopy showed that the P grain boundary monolayer coverage was virtually unchanged for all start-of-life, aging and irradiation conditions. In contrast, the C coverage reduced by 50% during the initial aging or irradiation, but changed little after longer irradiations. Grain boundary N increased initially but then stabilised. No grain boundary S occurred. Transmission microscopy revealed some dislocation loops and small regions of strain contrast in the grains after irradiation but no P-rich precipitates were seen. From this, and other published work, it is concluded that irradiation hardening arises from dislocation loops and possibly from P-rich cluster formation. Embrittlement developed due to loss of C from the grain boundaries, rather than from changes to the boundary P content. Loss of C reduced the boundary fracture stress but embrittlement was independent of the matrix strength. For the irradiation conditions examined, there was no evidence of irradiation-induced grain boundary P segregation.