The effects of temperature and moisture on thermal and mechanical properties of high-temperature cyanate ester composite materials were investigated. A resin transfer molding process was used to impregnate glass fiber fabrics with matrices that underwent thermoplastic or elastomeric toughness modifications. The elastomer-modified material obtained the highest mode I fracture toughness values primarily because the toughener did not phase separate. Extended exposure to 200 degrees C, however, deteriorated initial toughness improvements regardless of the modifier utilized. Although the thermal stability was increased by using thermoplastic modifiers in comparison to the elastomer-modified material, the degradation was mainly governed by the cyanate ester network. Gaseous degradation products caused delaminations and therefore reduced strength when the materials were exposed to 200 degrees C: for 1000 h, Also, upon immersion in water at 95 degrees C, the matrices absorbed up to 3.3 wt % more than previous values reported in the literature. Fiber/matrix interfacial phenomena were responsible for this behavior because fiber/matrix adhesion also was reduced drastically as shown by the strong reduction in flexural strength.