Poly(ethylene terephthalate) (PET) films possessing extremely enhanced hydrolytic stability were developed by combining the improved processes of polymerization, melt extrusion, and biaxial stretching. The PET films obtained from the combined process were better than commercially available PET films with highest hydrolytic stability at present. An accelerated test (at 120 degrees C and 100% relative humidity) was performed to learn the key parameters governing elongation-based hydrolytic durability of PET films. The results suggested that ultimate hydrolytic stability was because of the combined effects of significant decrease in the content of carboxylic acid terminal groups and enhanced crystallinity. Another reason for this was an increase in ground-state dimer sites due to intermolecular stacking between terephthalate units. These dimer sites were probably located in the densely packed amorphous regions, as suggested from the intrinsic fluorescence spectrum of PET films. The PET films developed in this study are very useful in outdoor applications such as backsheet materials in solar cell modules. POLYM. ENG. SCI., 58:261-271, 2018. (c) 2017 Society of Plastics Engineers