2,2-Bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)propane (BA-a) is blended with oligomers of polyarylsulfone (PSU) and polyarylethersulfone (PES) of different low/intermediate molecular weights (3000-12 000 g mol(-1)) and terminal functionality (chloro-, hydroxyl- or benzoxazinyl-(Bz)). Fracture toughness (K-IC) is observed to increase from 0.8 MPa m(0.5) for cured BA-a to 1 MPa m(0.5) with the incorporation of 10 wt % PSU-Bz (12 000 g mol(-1)). Generally, greater improvements in K-IC are observed for the PES olig-omers compared with the PSU oligomers of equivalent molecular weight. The terminal functionality of the thermoplastic has a lesser effect on improving toughness than increasing the molecular weight or the nature of the polymer backbone. Surface analysis of the fractured surfaces show greater phase separation and crack pinning in the PES toughened system. Where crack pinning is less obvious, as in the case of hydroxyl-terminated PES (of 6000 g mol(-1)), this coincides with a drop in fracture toughness.