Multiwall nanotubes that have undergone acidic treatment of varying duration were melt-compounded with poly(ethylene terephthalate) (PET) in a batch-mixer in order to create nanocomposite structures. The samples were pressed into films and annealed above the crystallization temperature of the polymer. Fourier Transform infrared (FT-IR) as well as Raman spectroscopies were utilized to monitor the gauche to trans transformations due to the annealing process with respect to the duration of the treatment of nanotubes. In addition, differential scanning calorimetry (DSC) measurements were performed to study the crystallinity changes due to the presence of the tubes. Acid-treated nanotubes were found not only to increase PET's crystallinity but also lead to a much better crystalline formation of the polymeric chains. The spectroscopic measurements in correlation with the DSC findings showed that neat PET has relatively more trans conformers in the noncrystalline phase than the PET-MWNT nanocomposite samples. Therefore, the presence of the nanotubes transforms the trans conformers of the noncrystalline phase into crystalline domains with the nanotubes acting as moderate nucleation agents.