The microstructure and crystallization behavior of a set of poly(ethylene terephthalate-co-5-nitroisophthalate) copolymers (PETNI) containing 5-nitroisophthalic units in the 10-50 mol % range were examined and compared to those of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate-co-isophthalate) (PETI) copolymers. A C-13 NMR analysis of PETNI copolymers in a trifluoroacetic acid solution indicates that they are random copolymers with average sequence lengths in accordance with ideal polycondensation statistics. Differential scanning calorimetry (DSC) studies show that PETNI containing 5-nitroisophthalic units up to 20 mol % are able to crystallize and that crystallization takes place in these copolymers at much slower rates than in PET. Wide-angle X-ray diffraction from powder and fibers reveals that crystallizable PETNI adopts the same triclinic crystal structure as PET, with the nitroisophthalate units being excluded from crystallites. Fourier transform infrared in combination with cross-polarization/magic-angle spinning C-13 NMR spectroscopy demonstrates the occurrence of a gauche-trans conversion encompassing the crystallization process. A correlation between DSC and spectroscopic data leads us to conclude that the content of trans conformer in the noncrystallized phase of PETNI is higher than in both PET and PETI copolymers and suggests that secondary crystallization in the homopolymer must proceed by a mechanism different than that in copolymers.