Poly(ethylene terephthalate) samples with volume fraction levels of crystallinity up to 27% have been evaluated in terms of physical aging, tensile mechanical properties, and environmental stress cracking failures. Specific enthalpy recovery values were found to correspond with transitions from ductile to brittle modes of tensile mechanical failures, exhibited by samples with each level of crystallinity. For samples exposed to aging temperatures from 45 to 60 degrees C, these critical enthalpies decrease linearly with increased crystallinity. The environmental stress cracking behavior of semicrystalline PET was found to depend on both crystallinity levels and physical aging. Samples with higher levels of crystallinity undergo stress cracking failures at lower critical stresses than their less crystalline counterparts. Physical aging, before environmental stress cracking exposure, further decreases these critical stress values and leads to much shorter critical times for failures. It has been shown that the interspherulitic amorphous portion of the material is primarily responsible for the differences in tensile mechanical and environmental stress cracking behaviors, observed as a result of aging and exposure. This amorphous region becomes progressively more restricted with increasing crystallinity as well as with increased aging. It, therefore, requires shorter exposure times to exhibit brittle tensile mechanical failure and environmental stress cracking rupture. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 2906-2917, 2009