Solution ring-chain reactions of poly(butylene terephthalate) (PBT) were carried out using 1,2-dichlorobenzene (b.p. 180 degrees C) with a catalyst. By increasing the dilution to 1:70 (polymer/solvent ratio w/v), cyclic PBT oligomers were obtained in a yield of 70 wt%. This compares to cyclic oligomers of PBT extracted from commercial PBT chip using mixed xylene isomers, in a yield of 1 wt%. The cyclic oligomers produced in the solution ring-chain reaction and obtained from the extract were found to contain oligomers (CO.C6H4CO.O.(CH2)(4)O)(x), where x = 2-9. The cyclic oligomers of PBT were analysed using PL-gel mixed E gel permeation chromatographic columns, fast atom bombardment mass spectrometry and liquid chromatography tandem mass spectrometry. The cyclic dimer of PBT, (CO.C6H4CO.O.(CH2)(4)O)(2), was separated, purified and crystallized and its X-ray crystal structure determined.