Crystal structures of the alpha and beta modifications of poly(tetramethylene succinate) (PTMS) were analyzed by X-ray diffraction: the beta form appeared with an application of stress. These two modifications belonged to the monoclinic system with the space group of P2(1)/n. In both cases, a unit cell included two chemical repeating units. For the alpha form, the cell dimensions were a = 0.523 nm, b = 0.912 nm, c (fiber axis)= 1.090 nm, and beta = 123.9 degrees; for the beta form, a = 0.584 nm, b = 0.832 nm, c (fiber axis)= 1.186 nm, and beta = 131.6 degrees. The difference in the fiber periods of the two crystalline forms was attributed mainly to the conformational difference in the tetramethylene unit, i.e. TGT (G) over bar T of the or form and TTTTT of the beta form. It was also found that in PTMS, the packing coefficient, K, which was defined by the ratio of the intrinsic volume with respect to the true volume of the alpha form was almost equal to that of the beta form. This observation could be contrasted to those obtained in poly(butylene terephthalate) (PBT), where the K of the alpha form was considerably greater than that of the beta form. The difference between PTMS and PET was attributed to the difference between the unit cell volumes of the alpha and beta forms of these polymers. (C) 2000 Elsevier Science Ltd. All rights reserved.