Reflectance infrared spectroscopy has been carried out for quantitative structural analysis of polyimide films thinner than 1000 Angstrom. Reflection spectra have been simulated for isotropic polyimide films of specific thickness on a metallic substrate. Structural analysis was based on a number of infrared active vibrations particularly sensitive to changes in chain conformation or packing. The structure of these films was compared with that obtained by annealing experiments on free standing films and crystalline powders. For adsorbed films with thicknesses <150 Angstrom, very low packing order was found. Frequency shifts in films of varying thicknesses were observed to be sensitive to changes in chain conjugation which are dependent on the coplanarity of rings along the backbone. Utilizing vibrations of known transition moment direction allowed one to characterize the degree of segmental orientation relative to the surface normal of variously prepared films. Spectroscopic data also revealed changes in orientation of the rigid planar structures in PMDA-ODA polyimide with respect to the surface normal upon variation of film thickness. The imide ring, preferentially aligned perpendicular to the surface normal in thin films, changes toward an isotropic orientation distribution. The chain orientation reached a constant value of f = -0.28. These spectroscopic studies were corroborated by electron diffraction tilting experiments obtained from flat films lifted from their respective gold substrates.