A study of hygrothermal aging in terms of the kinetics of moisture absorption by poly(butylene terephthalate) (PBT) and styrene-acrylonitrile/acrylate based core-shell rubber (CSR) toughened PET (PBT-CSR) was undertaken. The diffusion of water into the PET compounds with various CSR contents was investigated by immersion of specimens in water at temperatures between 30 and 90 degrees C. It was observed that the equilibrium moisture content and the diffusion coefficient of the PBT both increased with increasing CSR content. The fracture behaviors of the PET and PBT-CSR were investigated. The focus of investigation was on the effect of an internal parameter (rubber content) and external parameters (testing temperature, deformation rates, and hygrothermal aging) on the fracture behavior of these materials. The fracture,response of the various materials was evaluated by the fracture toughness and energy measured on static-loaded compact tension specimens. The tensile and fracture behavior of PET and PBT-CSR was affected by both the internal and external parameters. On its own the CSR impact modifier failed to improve the toughness of PET at either high testing speed or subambient temperature (-40 degrees C). Based on the dynamic mechanical analysis study, the CSR is believed to behave as a rigid particulate filler in the PET that consequently reduces the ductility of the PBT. All the materials tested showed poor retention of the tensile and fracture properties upon exposure to hygrothermal aging at 90 degrees C, and these properties could not be restored by subsequent drying. This was attributed to severe hydrolytic degradation of the PET that caused permanent damage to the materials. The failure modes of PET and PBT-CSR were assessed by fractographic studies in a scanning electron microscope.