The crystal structure of the mesophase polymer poly(ethylene 4,4’-biphenyldicarboxylate), [-C6H4-COO-CH2CH2-OOC-C6H4-]n. (P2BP), was established using the model compound approach. The model compound, diethyl 4,4’-biphenyldicarboxylate (2BP2), belongs to the triclinic system (a = 6.0625(2), b = 6.7033(3), c = 9.8917(3) angstrom; alpha = 97.62(1), beta = 99.82(1), gamma = 101.30(1)degrees), space group P1BAR. The structure of 2BP2 was solved by direct methods and refined by a least-squares procedure to R = 5.60%. The 2BP2 molecule is centrosymmetric with the side chain in the trans conformation and a disordered aromatic ring system. The structure of the parent polymer, P2BP, was deduced from its X-ray fiber pattern. P2BP has a triclinic unit cell (a = 5.75, b = 3.82, c = 14.62 A; alpha = 90.1, beta = 90.3, gamma = 78.1-degrees), and the space group is P1BAR. The geometrical and conformational parameters derived from 2BP2 were used to generate an initial polymer chain which was then packed into its unit cell. The chain position and orientation were then refined. The final R value reached 12.4%. The P2BP polymer adopts a nearly fully extended conformation. The biphenyl moiety is planar, while the carboxylate plane is tilted by 5-degrees with respect to the biphenyl plane. In the packing of both 2BP2 and P2BP, the chains are parallel to one another and the aromatic rings are face-to-face. However, in 2BP2, the molecules are displaced along the main molecular axis, while in P2BP, the chains are shifted sideways. It is suggested that, between 337 and 367-degrees-C, i.e., in the mesophase, P2BP is nematic with a face-to-face disposition of the rings.