This study forms part of a more general program addressed to developing a better understanding of the mechanisms of interfacial bonding and environmental stability in polymer matrix composites. Several published studies have shown that exposure of graphite fiber reinforced epoxy or polyester composites to water immersion or water vapor at elevated temperature produces decreases in interlaminar shear strength accompanied by rising or falling values of fracture toughness. An approach to the correction of this moisture sensitivity has been suggested by Kaelble based upon a-modified Griffith-Irwin model for interfacial failure and a micro-mechanics model for fracture energy due to fiber debonding.