A fracture mechanics approach has been used to investigate how the addition of high-temperature resistant thermoplastic modifiers can increase the fracture energy, G, of brittle cyanate-ester polymers. Tests were performed using adhesive joint specimens at -55, 21 and 150degreesC. The fracture energies of poly(ether sulfone)- and polyester-modified cyanate-ester polymers have been measured and scanning electron microscopy has been used to identify the toughening mechanisms involved. Firstly, it is shown that the addition of 25% by weight of a functionalised poly(ether sulfone) (PES) can increase the fracture energy of the cyanate-ester polymers by more than 800%, with the development of a particulate morphology within the modified polymer. Secondly, it is demonstrated that the use of 20% by weight of a polyester-copolymer elastomer can lead to an increase in the fracture energy of up to ten times that of the unmodified cyanate-ester polymer value. Thirdly, it is shown that several of the chemically-modified polymers developed and tested in the present work have fracture energies which are greater than a typical commercially-available cyanate-ester film adhesive. Finally, the effects of long-term ageing in air at 150degreesC, for up to 7500 hours, on the properties of unmodified, physically- and chemically-modified cyanate-ester polymers are investigated. It is shown that the measured fracture energies of some of these modified cyanate-ester polymers show no significant effect of such long-term ageing. (C) 2003 Kluwer Academic Publishers.