Polarization modulation infrared linear dichroism ( PM- IRLD) and ultrarapid- scanning Fourier transform infrared spectroscopy ( URS-FTIR) have been used to characterize the evolution of molecular orientation and microstructure during and following the step deformation of amorphous poly( ethylene terephthalate) ( PET) above and below its glass transition temperature. The combined use of these techniques allowed a high sensitivity and an unprecedented 10 ms time resolution for the characterization of irreversible polymer deformation using infrared spectroscopy. PM- IRLD results show that the 1410 cm(-1) band of PET, often used as a thickness standard, presents a significant dichroism even at low draw ratios. Using this band, the relaxation kinetics of the phenyl ring was directly shown, for the first time, to be similar to that of the glycol group in amorphous PET. These results suggest that the relaxation proceeds mainly via cooperative motions involving at least one repeat unit and not only through rotations around the flexible CH2- CH2 and CH2-O bonds. The real- time study of the cold drawing of glassy amorphous PET by URS- FTIR showed that a large gauche- to- trans conversion ( from similar to 15% to 60% of trans conformers) of the glycol groups occurs during the neck propagation. These trans conformers possess a very large and stable molecular orientation. Nevertheless, spectral analysis revealed that the " mesomorphic" phase, rather than the truly ( all- trans) crystalline structure, is produced during cold drawing of PET at room temperature.