Polyesters prepared from adipic acid (AA), phthalic anhydride (PA), isophthalic acid (IPA), terephthalic acid (TPA), and 2-butyl-2-ethyl-1,3-propanediol (BEPD) were investigated by gel permeation chromatography (GPC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry(MALDI-TOFMS). All polyesters were found to contain a fraction of low molecular weight molecules that were identified as cyclic ester oligomers. The most abundant cyclic oligomer varied with the diacid used in the synthesis but, in every case, it was either a dimer or a trimer. In addition, the relative amounts of the polyester starting materials influenced the amount of cyclic oligomers formed in the polyesterification reaction. Copyright © 2001 John Wiley & Sons, Ltd.