V-notched and fatigue precracked Charpy specimens of various sizes, tested in impact and slow bend, were used to study tempered martensite embrittlement in a 4340 steel. When plotted as a function of tempering temperature, the results showed that the magnitude of the toughness decrease caused by embrittlement varied with the type and size of the specimens. Embrittlement was always detected using thin samples but its detection in thick specimens depended on whether or not they contained a precrack. In particular, no embrittlement-associated fall in toughness was observed using standard size precracked samples tested in slow bend. Separation of the shear and flat fracture components of the absorbed energies showed that the variation of shear energy is a major factor contributing to embrittlement. The results are interpreted as indicating that intergranular fracture occurs more as the result of inhibition of plastic flow within the grains rather than directly as the result of the appearance of a low resistance crack path at the grain boundaries.