Extensive studies of the effect of alloying elements and residual elements on the susceptibility of Ni-Cr-Mo-V steels to temper embrittlement have led to the selection and evaluation of a preferred composition for a weldable quenched and tempered plate steel that is resistant to temper embrittlement during stress relieving. The embrittling effects of carbon, manganese, nickel, chromium, and molybdenum were determined in a statistically selected series of 27 composition modifications of a 5Ni-Cr-Mo-V steel. In addition, the effects of vanadium contents up to 0.19 per cent were investigated. The results of these studies showed that manganese and molybdenum were alloying elements primarily responsible for temper embrittlement in the steel. The effect of the residual elements phosphorus, aluminum, nitrogen, and oxygen on the susceptibility of 5Ni-Cr-Mo-V steel to temper embrittlement was determined. This study showed that although oxygen was detrimental to the notch toughness of the steels, only phosphorus contributed to temper embrittlement. On the basis of the preceding studies, a quenched and tempered plate steel with minimum susceptibility to temper embrittlement was selected; this composition exhibited good weldability and sufficient hardenability for 4-in.-thick plates. An 80-ton heat of this composition was produced, and the susceptibility of this steel to embrittlement during stress-relieving treatments was compared with that of a standard-composition 5Ni-Cr-Mo-V steel. The two steels were compared with and without 1 per cent prestrain. The modified-composition 5Ni-Cr-Mo-V steel exhibited about half the embrittlement of the standard composition. Embrittlement increased as the stress-relieving time increased and as the cooling rate from the stress-relieving temperature decreased. Prestraining produced no significant changes in the response of the steels to embrittlement.