A low percentage of cyclic oligomers can be found in poly(ethylene terephthalate) (PET) from its synthesis onward. In this article, a cyclic-oligomer-fr ee PET (COFP) obtained by solvent extraction was used to study the thermodynamics of the re-formation of cyclics from the melt. The cyclic-oligomer content re-increased into molten COFP, finally reaching an equilibrium. An analysis of the fraction of the re-formed cyclic oligomers showed that a majority of cyclic trimer (60-70%) was found at the equilibrium Before the establishment of the equilibrium, an unusual behavior was observed in the relative proportion of cyclic trimer and tetramer during the first steps of their formation that was probably due to a competition between kinetic and thermodynamic products.