Biaxial orientation in ultrathin films is characterized by a preferential orientation of the main molecular axis in the plane of the film in addition to its orientation with respect to the normal of the film. In this paper, analytical expressions allowing the calculation of the molecular orientation in biaxially oriented films from their anisotropic optical constants are presented for the first time. These expressions have been used to calculate the molecular orientation in multilayers of poly(gamma-benzyl-L-glutamate) and to determine the angles between the main molecular axis and the different transition moments used (alpha angles). Using the optical constants determined by Buffeteau et al.(1) for PBG multilayers, we have found that the tilt angle of the PBG helices with respect to the monolayer normal is 89degrees and that the azimuthal angle in the plane is 57degrees. The calculated alpha angles for the carbonyl stretching mode of the ester group and the amide I and amide II modes of the amide group are 49, 34 and 74degrees, respectively. The analytical expressions presented in this paper have also been used to calculate the molecular orientation in Langmuir-Blodgett monolayers of an equimolar blend of poly(D-lactide) and poly(L-lactide). The optical constants of a monolayer of the polylactide mixture determined using p- and s-polarized transmittance and p-polarized reflection-absorption infrared spectra have shown that the polylactide helices have a tilt angle of 81degrees and an azimuthal angle of 56degrees.