Auger electron spectroscopy has been used to examine the fracture surfaces of specimens of W-Ni- Cu and W-Ni-Fe particulate composites in both the as-sintered condition and following post-sintering heat treatments. Changes in the levels of impurity contamination on the fracture surfaces and changes in fracture mode detected by, scanning electron microscopy have been correlated with changes in toughness determined by comparative impact tests. Changes in microstructure accompanying heat treatments have also been monitored using both quantitative metallography' and transmission electron microscopy. Brittle failure of as-sintered, furnace cooled specimens occurs principally by interfacial decohesion and appears to be associated with the presence of an excess concentration of phosphorus, and possibly sulphur, at the interphase boundaries. High temperature solution treatment followed by quenching produces a reduction in the level of segregation, an improvement in the interphase boundary strength, and a resultant improvement in impact resistance.