A series of C-Mn pressure vessel plates and other steels containing various levels of copper, phosphorus and boron were irradiated at 240°C in two differing neutron spectra in the Halden reactor. Hardening and embrittlement data obtained after exposure in a highly thermalised neutron spectrum with dpathermal(>1keV)/dpafast(<1keV) = 2 were compared with data obtained after exposure to a cadmium-shielded spectrum with dpathermal/dpafast = 0.01. Exposure times varied from 85 to 251 days at a mean fast dpa dose rate of 6 × 10-11 dpa/second. For C-Mn plate steels containing low levels of impurity boron, the respective shifts in yield stress and Charpy transition temperature were always greater after exposure to the thermalised spectrum when compared with exposures to the cadmium-shielded spectrum. The shifts in the cadmium shielded spectrum agreed with data trend predictions from many surveillance and accelerated irradiations where thermal dpa doses were small. The influence of thermal neutrons was incorporated into trend curves by using an “effective” dpa dose, i.e. dpaeffective= dpafast + k dpathermal where k is a thermal neutron efficiency factor. For doses <300 × 10-5dpathermal the value of k was 2.0 ± 0.5. In a C-Mn steel containing an increased level of impurity boron, an allowance was required for the additional dpa generated by the thermal neutron-induced transmutation of boron-10. For thermal doses where the burn-out of boron-10 was not too great, the value of k in the embrittlement equation increased to 2.0 + 0.3(B-1) ± 0.5 for steels containing B weight ppm of natural boron. For a high copper C-Mn-Mo steel, hardening and embrittlement shifts arose due to irradiation-enhanced copper precipitation. Over the thermal dose range examined, thermal neutrons did not change the dose dependence of copper precipitation hardening.