Display omitted ► Vibrational spectrum of PET complicated by the presence of rotational isomers and changes on crystallization. ► Synchronous and asynchronous correlation spectra enables the absorption bands to be unambiguously assigned. ► Fractional crystallinity can be directly determined from the amorphous and crystalline band ratio. The vibrational spectrum of partially crystalline poly(ethylene terephthalate), PET, appears to be unduly complex in that the absorption bands are split into amorphous and crystalline modes and are sensitive to chain configuration and orientation. Assignment of the bands has accordingly proved to be difficult and lead to differences in interpretation. Two-dimensional infrared spectroscopy is a recent novel analytical technique in vibrational spectroscopy which can be used to interpret differences in the spectra with time or temperature accompanying changes in structure and morphology and has been used to analyze the development of crystallinity within a polymer in which amorphous regions are transformed into crystalline regions and involving changes in molecular configuration. Two-dimensional IR correlation spectroscopy has been successfully applied to these problems in the attempt to measure the fractional extent of crystallinity in PET as a function of time and crystallization temperature as well as changes to molecular configuration.