Long-term creep tests on poly(ethylene terephthalate) (PET) films were conducted at different temperatures and humidities. Shift factors were calculated using time-temperature and time-humidity superpositions. We found that the activation energy of PET films appears to be constant at the test conditions (below its glass-transition temperature). Using the temperature and humidity shift factors, the shift factor at any temperature and humidity can be predicted. Tests were conducted to study the viscoelastic behavior of PET films subjected to a time-dependent temperature field. Tests were also conducted to compare the viscoelastic behavior of coated tapes and uncoated PET substrates.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Tribology Conference in Atlanta, Georgia, October 8–10, 1985