New cyclic oligomers of the copolymer of poly(ethylene terephthalate) (PET) and poly(ethylene isophthalate) (PEI) were isolated and identified. A condensation polymerization was carried out at a high temperature, and the solid-state polymerization that followed yielded the high molecular weight polymer. The oligomers were extracted from the high molecular weight PET-PEI copolymer and separated with preparative high performance liquid chromatography techniques. Their chemical structures and properties were analyzed and determined by H-1 NMR, differential scanning calorimetry, and mass spectroscopy. The oligomers observed at early retention times were a cyclic dimer and cyclic trimers and consisted of [GT](3), [GI](2), [GI](3), [GT](2), [GI](1), and [GT](1)[GI](2). (C) 2003 Wiley Periodicals, Inc.