Cyclic oligomers of tetraethylene glycol terephthalate (TEGT), tetraethylene glycol isophthalate (TEGI), tetraethylene glycol orthophthalate (TEGO), decamethylene terephthalate (DMT), decamethylene isophthalate (DMI) and decamethylene orthophthalate (DMO) were prepared successfully and cleanly from a ring-chain equilibrium reaction in dilute solution of chlorobenzene. The reactions were monitored by gel permeation chromatography (GPC) and proton nmr and C-13 NMR spectroscopy, and the molar cyclic concentrations [M-x] deduced (x = 1-10). The cyclic oligomers were also analysed by fast atom bombardment mass spectrometry (FAB-MS), matrix assisted laser desorption ionization-time of Right mass spectometry (MALDI-TOF MS) and differential scanning calorimetry (DSC). The existence of the cyclic oligomers was proven by all of the above methods. In addition, it was found that small amounts of linear species were formed, especially in the TEGT system.