Embrittlement of low-alloy steel components used in the nuclear power industry is classified into hardening embrittlement and non-hardening embrittlement. Hardening embrittlement stems from precipitation of carbides or copper-rich compounds. Non-hardening embrittlement results mainly from grain boundary segregation of phosphorus. To examine the non-hardening embrittlement in 2.25Cr 1Mo steel, phosphorus intergranular segregation in samples subjected to neutron irradiation and thermal aging at 270°C and 400°C is determined by modelling and transmission electron microscopy. The ductile-to-brittle transition temperature for the steel is determined by means of small punch testing with 3 mm diameter disc specimens. It is indicated that there is a reasonable agreement between the behavior of intergranular phosphorus segregation and shifts in the ductile-to-brittle transition temperature. Measured and predicted phosphorus intergranular segregation in a range of pressure vessel steels is also discussed.