Molecular modelling and X-ray structure studies were undertaken to determine the crystalline unit cell of LARC-CPI, a thermoplastic semicrystalline polyimide. We present results of a molecular mechanics based model of the energy minimized unit cell structure. The unit cell of LARC-CPI has lattice parameters of a = 8.0, b = 6.0, c = 37.1 Angstrom, alpha = beta = gamma = 90 degrees as determined from the model. The model unit cell contains two monomer units, one at the centre of four equivalent monomers. This gives a cell density of 1.47 g cm(-3) which compares well with the experimentally determined unit cell density of 1.507 +/- 0.2 g cm(-3). Fibre and powder X-ray diffraction patterns were constructed based on the model unit cell. To compare the model diffraction patterns with experimental data, a novel preparation method developed in earlier work was used to obtain highly oriented LARC-CPI films. Films recrystallized by solvent exposure at the reflux temperature had c-axes preferentially aligned along the film normal. The c-axis repeat length was determined experimentally to be 37.5 Angstrom in agreement with the model value. X-ray photoelectron spectroscopy was used to identify the origin of a unique, radiation sensitive diffraction peak observed in some LARC-CPI films. This peak could not be indexed using the proposed model unit cell parameters. We suggest that this peak arises from oriented crystalline diamines, or diamine-terminated chains, located at the film surfaces.