Structure and properties of commercially available fully oriented thermoplastic and thermotropic polyester fibers have been investigated using optical birefringence, infrared spectroscopy, wide-angle X-ray diffraction and tensile testing methods. The effect of the replacement of p-phenylene ring in poly(ethylene terephthalate) (PET) with stiffer and bulkier naphthalene ring in Poly(ethylene 2,6-naphthalate) (PEN) structure to result in an enhanced birefringence and tensile modulus values is shown. There exists a similar case with the replacement of linear flexible ethylene units in PET and PEN fibers with fully aromatic rigid rings in thermotropic polyesters. Infrared spectroscopy is used in the determination of crystallinity values through the estimation of trans conformer contents in the crystalline phase. The analysis of results obtained from infrared spectroscopy data of highly oriented PET and PEN fibers suggests that trans conformers in the crystalline phase are more highly oriented than gauche conformers in the amorphous phase. Analysis of X-ray diffraction traces and infrared spectra shows the presence of polymorphic structure consisting of alpha- and beta-phase structures in the fully oriented PEN fiber. The results suggest that the trans conformers in the beta-phase is more highly oriented than the a-phase. X-ray analysis of Vectran((R)) MK fiber suggests a lateral organization arising from high temperature modification of poly(p-oxybenzoate) structure, whereas the structure of Vectran((R)) HS fiber contains regions adopting lateral chain packing similar to the room temperature modification of poly(p-oxybenzoate). Both fibers are shown by X-ray diffraction and infrared analyses to consist of predominantly oriented noncrystalline (63-64%) structure together with smaller proportion of oriented crystalline (22-24%) and unoriented noncrystalline (12-15%) structures. (c) 2006 Wiley Periodicals, Inc.