A general technical approach for characterizing polymer composite reliability has been applied to two 177°C (350°F) service ceiling graphite-epoxy materials. A comparison of the chemical composition and environmental durability of NARMCO 5208/T300 and FIBERITE 934/T300 has been made. Chemical analysis of these resins by infrared spectroscopy, liquid chromatography, atomic absorption spectroscopy, and differential scanning calorimetry indicates a common epoxy resin and amine cross linker, but differences as to the presence of boron catalyst and reactive resin additives. These latter modifiers are shown to enhance processability, but also to influence thermal response and environmental durability of the cured composite. Nondestructive ultrasonic attenuation measurements transverse to the fiber axis correlate with microcracking while acoustic wave velocity is sensitive to current moisture content. These physiochemical measurements form the basis for a systematic program of composite durability which encompasses matrix chemistry and bulk properties, interfacial bonding mechanisms, composite environmental response, and fracture mechanics.