<,正,>, Three specimens from a solution-cast poly （ethylene terephthalate） （PET） film, one being liquid-N,2, quenched from 92℃（Q）, one being slowly cooled from 92℃（SC） and one being quenched and sub-T,g, annealed at 67℃（AN）, have been studied by specimen difference spectra Q-SC and AN-Q and temperature difference spectra T-70 and T,2,-T,1, for every 2℃steps on heating to 90℃at 2℃/min. SC and AN showed more gauche conformers than Q. That means that the PET chain has more trans conformers at higher temperatures and some of these are frozen during quenching through T,g,. A band at 1340 cm,-1, has been found to be complex containing overlapping bands reflecting trans in crystalline regions and trans in amorphous regions. The temperature difference spectra on heating through T,g, showed that the spectral changes in Q are gradual while a rather abrupt change occurs in AN at 80—82℃for the bands at 1340, 1042 and 1020 cm,-1,. No new conformational structure or new vibrational mode is involved. A kind of locking mechanism is suggested which hinders the molecular vibrational changes in AN below T,g, until a sudden release occurs at T,g,. These locking sites can be nothing else than sites of tighter local packing of chain segments. Consequently it is believed that inter-chain van der Waals attraction energy plays a dominating role in the volume relaxation and sub-T,g, annealing of quenched amorphous polymers.