Intermolecular interchange reactions in poly(ethylene terephthalate) (PET)/poly(butylene terephthalate) (PBT) blend system were investigated to reveal the changes of chemical structure and properties of the blends. The copolymers formed by the interchange reaction between PET and PBT were separated and analyzed. The compositions and molecular weights of PET-PBT copolymers were examined by proton nuclear magnetic resonance spectroscopy and viscometry, respectively. Also, the sequence length of the interchange-reacted PET/PBT blend was measured by carbon-13 nuclear magnetic resonance spectroscopy, The solubility of the interchange reaction product of PET/PBT blend increased with reaction time and was the highest at the equivalent composition of blend. As the sequence length of reaction product decreased, the solubility increased. The reactivity of butylene terephthalate unit was slightly higher than that of ethylene terephthalate unit. Molecular weight of the copolymer increased initially but decreased thereafter.