In the last years, a significant amount of research is being performed in the field of polymer research for novel applications, such as flexible electronic devices, photovoltaic cells, high performance optics, data storage, etc. Toward this direction, in this work, the optical anisotropy of biaxially stretched poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) films has been extensively investigated. The optical properties of the films have been studied in terms of their optical, electronic and vibrational response, by Fourier transform IR spectroscopic ellipsometry (FTIRSE) (900-3500 cm(-1)) and Vis-fUV variable angle SE (1.5-6.5 eV) techniques. The films optical anisotropy is the result of the stretching procedure during their fabrication, which results to the structural rearrangement of the macromolecular chains parallel to the stretching direction and to a higher structural symmetry. During the SE spectra analysis, the films have been approximated as uniaxial materials with their optic axis parallel to the sample/ambient interface leading to the accurate determination of the principal components epsilon(parallel to)(omega) and epsilon(perpendicular to)(omega) of the dielectric function e(w). The detailed study of the electronic transitions has been performed in the Vis-fUV region, where the characteristic features corresponding to the n -> pi* electronic transitions of the carbonyl-C=O group and the (1)A1g -> B-1(1u) transition due to the pi -> pi* excitation of the pi-electron structures have been identified and analysed. Furthermore, the FTIRSE spectra allowed the accurate identification and assignment of the features of epsilon(omega) to the vibrational modes of the various bonding structures characteristic of the PET and PEN macromolecular chains. (c) 2006 Elsevier B.V. All rights reserved.