Segregation of P and Mo to prior austenite grain boundaries in a 2.25Cr1Mo steel subjected to neutron irradiation and ageing is examined using field emission gull scanning transmission electron microscopy (FEGSTEM) with energy dispersive X-ray microanalysis. The steel samples are irradiated and aged at around 270 and 400 degrees C, respectively. The irradiation dose rate and dose are about 1.05 x 10(-8) dpa s(-1) and 0.032 dpa for 370 degrees C-irradiation, and 1.75 x 10(-8) dpa s(-1) and 0.13 dpa for 400 degrees C-irradiation. The FEGSTEM results indicate that, during 270 degrees C-irradiation, there is no irradiation effect for P segregation and some effect for Mo segregation with the Mo boundary concentration decreasing. During 400 degrees C-irradiation, there is a slight irradiation effect for P segregation with the P boundary concentration increasing and some irradiation effect for Mo segregation with the Mo boundary concentration decreasing. The 400 degrees C-ageing does not enhance Mo segregation as compared with the 270 degrees C-ageing, but it enhances P segregation. (C) 2000 Elsevier Science S.A. All rights reserved.