A detailed investigation has been carried out to study the effects of heat treatment on the susceptibility to temper embrittlement (Ductile-Brittle transition temperature and low energy fracture characteristics) of eleven experimental steels. These experimental alloys represented a range of compositions related to engineering steels and corresponded to a nominal composition of 0.34 mass%C with alloying additions of Cr, Ni and Mo in varying combinations. These alloys were doped with P and Sb to study the effect of these additions on the susceptibility to temper embrittlement. These steels were investigated in the pearlitic, bainitic and tempered martensitic conditions to establish the role of microstructure. Heat-treated alloys were characterised by fracture studies including Izod impact testing and subsequent electron microscopy. Simultaneous Auger electron spectroscopy and energy dispersive X-ray analysis coupled with with Secondary Ion Mass Spectrometry were used to study the fracture surfaces and bulk compositions of the embrittled structures. It has been established that intergranular embrittlement in a quenched and tempered martensite microstructure was associated with the presence of P, whereas the initial intergranular embrittlement in a bainitic microstructure was associated with the segregation of Sb. It is suggested that the lower C activity produced in tempered martensite structures allows P migration to the grain boundaries causing intergranular embrittlement which was attributed to the development of M[7]C[3].Alloys in isothermally transformed bainitic condition showed that the predominant carbide precipitate was M3C which increased the C activity at the prior austenite grain boundaries with a resultant decrease in P concentration and consequently an absence of intergranular failure in the early stages of embrittlement. The increased C activity continued to prevent appreciable P segregation but was not sufficient to inhibit the co-segregation of Ni and Sb after extended ageing times when the bainitic alloys began to fail by intergranular fracture. After prolonged ageing increased Ni and Sb concentrations at the grain boundaries were associated with the formation of a fine grain boundary precipitate which was low in Cr. The tendency to fail by the low energy intergranular mode of failure was always greater in the tempered martensites, even when the bainites were significantly harder.