Studies were conducted to establish the effects of thermo-oxidative and hydrothermal exposure on the mechanical properties of T300 graphite fabric reinforced PMR-15 composites. The effects of hydrothermal exposure on the mechanical properties of HTS-2 continuous graphite fiber composites were also investigated. The thermo-oxidative stability characteristics of T300 fabric and T300 fabric/PMR-15 composites were determined. Flexural strengths of specimens from composites in the as-fabricated and environmentally exposed conditions were determined. The useful lifetime of T300 fabric/PMR-15 composites in air at 316°C was found to be about 100 h. The useful lifetimes in air at 228 and 260°C were determined to be 500 and 1000 h, respectively. Absorbed moisture was found to reduce the elevated temperature properties of both the T300 fabric and HTS-2 continuous fiber composites. The moisture effect was found to be reversible.