Summary: Cyclodi(ethylene succinate) (C2) easily reacts with poly(ethylene terephthalate) (PET) in the melt leading to the formation of high molar mass PET-Poly(ethylene succinate) copolymers (PET-PES). Copolyesters with a PET/C2 starting mass ratio of 90/10, 80/20, 70/30 and 50/50 were synthesized and characterized by 1H NMR and MALDI-TOF MS. The 50/50 copolyester was almost random, while copolyesters with higher ethylene terephthalate contents exhibited some block copolymer character. MALDI-TOF MS/SEC off-line coupling was used to determine copolyester absolute average molar masses. The results indicate that the conventional SEC polystyrene calibration strongly overestimates copolyester molar masses. The melting temperatures and crystallinity of the 90/10, 80/20 and 70/30 copolyesters were significantly higher than those of comparable PET-aliphatic polyester copolymers.