It has been observed that post weld heat treatrents designed to reduce or remove residual welding stresses, may cause weld metal eTnbrittlen. ent. In this investigation, the effects of post weld heat treatment on three high strength submerged are weld metals were examined in terms of changes in'mechanical properties, principally fracture toughness. Metallographic work was carried out to determine the cause of ezbrittlement. Post weld heat treatment was performed in the 450 to 650 0C temperature range for times up to 50 hours and embrittlement was meatured in terms of Charpy, COD and J contour integral tests. It was found that each weld metal could be enbrittled and that two distinct forms of embrittlement occurred. The first was due to alloy carbide precipitation which promoted trans-granular cleavage. Vanadium at 0.12% could cause severe embrittlement while molybdenum, at up to 0.6% did not. The second forn of embrittle-ment was classical temper embrittlement which caused low energy decohesion along prior austenite grain boundaries. This was caused by the migration of phosphorus to these boundaries during heat treatment and bulk phosphorus concentraticrs of 0.011% could cause severe erbrittlement. Embrittlement occurred in the 450 to 550 0C temperature range and increased with time and decreasing cooling rate. Marten-sitic microstructures were more susceptible than acicular ferrite. Defect tolcrance calculations based on COD-and J determinations showed that defect tolerance could increase after heat treatment despite-embrittlement although the final defect tolerance was strongly dependent on residual stress levels remaining, after heat treatment. Heat treatment procedures for the three weld metals examined were suggested, along with general guidelines for the post weld heat treatment of other high strength weld metals.