Carbon fibre preimpregnated tape (prepreg) was prepared from a range of prepolymers comprising several different blends of alkenyl-functionalised cyanate ester (CE) monomers with commercial CE and bismaleimide (BMI) monomers and blends. Carbon fibre composites were prepared by autoclave moulding. Dynamic mechanical thermal analysis (DMTA) was used to assess cure cycles by the measurement of glass transition temperatures (T-g). Mechanical properties are reported for near resins and compared with the corresponding composite properties such as interlaminar shear strength, flexural strength, compressive strength and mode I fracture toughness (Gic). BY incorporation of alkenyl-functionalised CE monomers into CE/BMI blends it is possible to raise the value of T-g while maintaining levels of G(IC) (when compared with an unmodified BMI/CE blend) and without leading to a degradation in other mechanical properties. This enhancement in neat resin fracture toughness was translated into the corresponding composite. For example, a novel modifier 5rPOCN, at an incorporation level of 15% by weight, produces a composite with T-g = 170 degreesC, interlaminar shear strength 113 MPa and G(IC) = 513 J/m(2), comparable with reported thermoplastic-toughened CE composites. Crown Copyright (C) 2000 Published by Elsevier Science Ltd. All rights reserved.