The thermal behavior of poly(hexamethylene terephthalate) (PHT) oligomers containing variable amounts of isophthalic (IPA) monomer has been investigated. Poly(hexamethylene terephthalate co-isophthalate) 3400 was first studied. This polymer has a <(M)over bar n> of 3400, is constituted by heterodisperse PHT units (crystallizable units) with a <(M)over bar n> of 2000, contains an average number of 2.6 IPA per chain, and has carboxylic chain ends. DSC results have shown that this polymer exhibits multiple melting endotherms. These are described in terms of (i) the existence of two melting zones located on both sides of T-c and associated with two polymer fractions differentiated by their crystallizable unit length, (ii) different crystalline populations growing at T-c, (iii) a reorganization process. A simultaneous small (SAXS) and wide (WAXS) angle time resolved X-ray scattering study using synchrotron radiation has revealed that the different crystalline populations are characterized by different lamellar structures rather than by different crystalline lattices. Other PHTcoIs and PHTs differing from PHTcoI 3400 by their molecular weight and TPA content were also studied using DSC. IPA acts as a defect for crystallinity and causes T-m and Delta H-m depression in the PHTcoIs. When the molecular weight of PHT increases, T-m increases and Delta H-m decreases as usually reported in the literature. Finally, two crosslinked polymers (PHTcoI 3400 and PHTcoI 4700) were investigated. The insoluble fraction of crosslinked PHTcoI 3400 does not crystallize anymore while the one of PHTcoI 4700 does. This results from shorter crystallizable units and from the distance between the crosslinks of the former.