The effect of alloy composition on the fracture toughness of Ti-Al-Nb alloys, which undergo stress-induced martensitic transformation (SIMT), was investigated for a given volume fraction of primary α2. Both Ti-18Al-8Nb and Ti-15Al-12Nb alloys exhibited lower fracture toughness values as compared to Ti-15Al-8Nb alloy, which indicated that higher Al or Nb was detrimental to fracture toughness in these alloys in the presence of primary α2. The results were explained on the basis of the effect of composition on fracture toughness contributions due to SIMT as well as ductile fracture. © The Minerals, Metals & Materials Society and ASM International 2009.