In a series of three papers we present the results of surface structure analysis of anisotropic polymer films using ATR-IR spectroscopy. In the first paper, the methodology for quantitative ATR-IR analysis of optically biaxial polymer films of orthorhombic symmetry is outlined, and results are presented of the first stage of the analysis-the extraction of the anisotropic optical constants of the films. Attention is drawn to the necessity that adequate sample/ATR-IR crystal contact is achieved in order to measure the correct reflectivity spectra of the sample/crystal interface and therefore obtain the correct (n, k) spectra of the analyzed film. The anisotropic absorption index spectra of PET and isotactic polypropylene films are presented in the 1800-670 and 1400-800 cm(-1) range, respectively, along with the ATR-IR spectra from which the optical constants have been calculated. The extraction of all three principal absorption index spectra of PET films allowed us to examine the optical anisotropy of PET films formed by uniaxial-planar and biaxial drawing. The uniaxial-planar films are strongly optically biaxial, whereas the biaxially drawn films have similar properties in the film plane, which are very different from those along the film thickness direction. These differences in optical anisotropy are related to the difference in chain orientation induced during uniaxial-planar and biaxial deformation of PET films and to the anisotropy of the PET monomer unit.