In this paper we provide an assignment of symmetries for all transitions observed in the infrared spectrum of phthalic anhydride (1a), based on data determined from polarization studies of photooriented 1a. Photoselection, using two electronic transitions in phthalic anhydride 1a, at 34083 and 40650 cm(-1) and of A(1) and B-2 symmetry, respectively, produced two differently oriented uniaxial samples of the anhydride immobilized in neon and other noble-gas matrices. Subsequent polarization measurements in the infrared region, combined with polarized Raman data, allowed a complete determination of transition moment directions for all observed fundamental vibrations. To facilitate definitive spectral assignments, we studied three isotopomers of 1a : 1,2-C-13-labeled phthalic anhydride (1b), the perdeuterated phthalic anhydride (1c), and the alpha-C-13-labeled compound (1d). Absolute infrared absorption intensities were determined for all observed transitions utilizing absolute infrared absorption intensities of CO and CO2, formed in equimolar amounts with o-benzyne (2) in phototransformations of 1, Interpretation of the experimental results and the final assignments were aided by quantum mechanical modeling at the SCF/6-31G** level.