The reasons for the improvement in the fracture toughness of an Fe-12 at% Ni base alloy at −196° C by the addition of small amounts of Ti were investigated employing transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Auger electron spectroscopy. Ti additions ranging from 0.18 to 0.99 at% and heat treatments of 2 h at 550, 685 and 820° C respectively followed by a water quench were considered, since previous work by Witzke and Stephens had shown that maximumKIC occurred for an Fe-12 at% Ni-0.18 at% Ti alloy heat treated at 685° C. It was found here thatKIC at −196° C for all the alloys and heat treatments correlated with the fraction of ductile fracture compared to intergranular and cleavage fracture, the latter modes being predominant in the Fe-12 at% Ni base alloy without Ti additions. Cubic and rectangular shaped inclusions were noted in the SEM fractographs of the alloys with the Ti additions. A fine precipitate was observed by TEM for the Fe-12 at% Ni-0.18 at% Ti alloy heat treated at 550° C; this precipitate was not observed for the 685 and 820° C heat treatments of the same alloy. Auger mappings of the fracture surfaces indicated a weak to moderate association of the interstitials C, N and O with Ti, the degree of which depended on the particular interstitial and the heat treatment temperature. It was concluded that the increase inKIC due to the initial 0.18 at% addition of Ti was due to a scavenging of interstitials which normally segregate at the grain boundaries and to the refinement of the microstructure. The subsequent decrease inKIC with further Ti additions was attributed to the increase in flow stress and slight lowering of the fracture stress resulting from these additions. It was further inferred that the Ti additions and heat treatment on the flow and fracture stresses may be due in a large part to their influence on the amount, size and distribution of the precipitate which was observed.