The molecular and crystal structure of the a form of poly (tetramethylene adipate) (PTMA) was analyzed using synchrotron X-ray fiber diffraction data. The crystals belong to the monoclinic system of space group P2(1)/n. The unit cell constants are a = 0.6776(6), b = 0.7904(6), c (fiber axis)= 1.442(1) nm and beta = 135.6(1)degrees. The final crystal structure was obtained by the linked-atom least-squares refinement, which gave an R-factor of 0.130 for 103 observed spots and 64 unobserved reflections. The molecular structure deviates slightly from the fully extended conformation in the ester part. The torsional angle CH2-CH2-O-C(=O) was found to be 155 degrees. The C=O groups of the corner and center chains in a unit cell are closely located along the c-axis and are related by the crystallographic 2(1)-axes along the b-axis at z = 1/4 and z = 3/4. The total dipole moment arising from the C=O groups is oriented in one direction at z = 1/4, and in the opposite direction at z = 3/4. Owing to the close arrangement of the C=O groups between neighboring chains along the fiber axis, the c-projected cell dimensions and the setting angle of the polymer chain are different from those of the orthorhombic form of polyethylene and the beta form of PTMA. (c) 2005 Elsevier Ltd. All rights reserved.