Cyclic oligomers [CO . C6H4 . CO . O . CH2 . CH2 . O](x) (with x = 3-8) of poly(ethylene terephthalate) have been prepared by a polymer-supported reaction. They have been investigated by field desorption mass spectrometry and liquid chromatography/mass spectrometry. The particular stability of the cyclic trimer x = 3 is demonstrated by the results.