We present a new method for the characterization of molecular orientation in polymer materials based on the determination of the optical anisotropy by reflectance difference spectroscopy (RDS). Data interpretation is quite straightforward even in the case of thin transparent films, as shown for the example of biaxially oriented poly(ethylene terephthalate) (PET). A comparison with birefringence data obtained by spectroscopic ellipsometry (SE) demonstrates the superior measurement precision and robustness of RDS. Using azimuth dependent RDS, the position of the optical eigenaxes in the film plane can be established, which are found to coincide with the crystalline orientation determined by wide-angle X-ray scattering. (c) 2006 Elsevier Ltd. All rights reserved.