Hydroxy-ester interchanges were found to be the predominant reactions involved in poly-(ethylene terephthalate)-poly(ε-caprolactone) copolyester randomization. Even for high molar-mass copolyesters having low hydroxyl end group concentration, kinetic studies show that ester-ester interchanges play a minor role. Reaction rate was almost proportional to hydroxyl end groups concentration. The randomization kinetics was studied by 1 H NMR on copolyesters obtained in situ by reacting a series of poly(ethylene terephthalate)s of various molar masses and end groups with ε-caprolactone, in the molten state and in the presence of Ti(OBu) 4 catalyst. A mechanism involving parallel catalyzed second-order hydroxy-ester and ester-ester interchanges is proposed for the randomization step and perfectly fits experimental data. This study clearly shows that the nature and concentration of hydroxyl end groups must be carefully controlled for reliable synthesis of copolyesters by homopolyester melt blending.