The physical aging characteristics of oriented poly(ethylene terephthalate (PET), have been studied as functions of storage time and temperature below the glass transition temperature (T-g) of PET. The free volume relaxation, associated with aging, has been characterized by the enthalpy of relaxation at T-g, as measured by differential scanning calorimetry. The effects of the free volume relaxation on mechanical properties and the mode of failure have been investigated. It has been determined that a correlation exists between the enthalpy of relaxation and the ductile-to-brittle failure transition. Molecular orientation reduces significantly the enthalpy of relaxation, resulting in the disappearance of the ductile-to-brittle transition when highly oriented samples are aged over time. It has been established that a minimum amount of orientation is required to reduce or eliminate the effects of PET aging. Molecular orientation has also been found to reduce craze formation when oriented PET is exposed to a stress-cracking medium at a constant stress.