Tracking the Raman scattering bands of polyethylene terephthalate (PET) during thermal heating has allowed to establish criteria for monitoring the crystallinity ratio within a few seconds. The existing criteria, based on Raman bands at 998 cm−1 and at 1096 cm−1, present certain disparities from the crystallinity ratio given by the total heat flow in differential scanning calorimetry (DSC). Based on the evolution of the Raman bands, a newfound method allowing the effective calculation of crystallinity ratio is proposed. For the same, the band at 1727 cm−1, which is characteristic of the C=O carbonyl bonds of the ester group of PET, is used. The new criterion, designated r1727,2, was compared and validated by DSC and XRD. This novel approach also provides access to information on the various conformational states of the bonds under study, that is, their proportions and positions with respect to the plane of the macromolecule aromatic rings. Evidently, this allows to reveal the characteristic thermal behavior of the PET microstructure such as the glass transition, crystallization, and melting. This method can be useful for the close detection, quantification, understanding, and tracing of the micromechanisms involved at the macromolecular conformations scale during thermal cycles. [ABSTRACT FROM AUTHOR]