Oligomeric extracts of poly(ethylene terephthalate-co-ethylene isophthalate)s [(PET/EI] are analyzed by high performance liquid chromatography(HPLC) and nuclear magnetic resonance spectroscopy (NMR). Existence of separated peaks for small cyclics of trimer and tetramer gives the existence of structural isomeric forms. NMR confirms that cyclization of PET/EI occurs more easily at the site of isophthaloyl unit.