An efficient. convenient and quantitative method for characterising polyester end-groups is described. We have found that trichloroacetyl isocyanate (TAI) reacts rapidly and quantitatively with both carboxyl [C(O)OH] and hydroxyl (OH) chain ends to form derivatives that can be readily determined by H-1 NMR spectroscopy. The TAI capped end-groups give rise to characteristic imidic NH resonances in a normally clear region of the H-1 NMR spectrum [delta similar to 10-11.5 for C(O)-O-C(O)-NH-C(O)CCl3 from C(O)OH, delta similar to 8-9 for O-C(O)-NH-C(O)CCl3 from OH]. The method has been successfully applied to quantitative determination of the end-groups of a wide variety of oligomeric polyesters. It has also been applied to higher molecular weight polyesters including commercial, bottle grade, poly(ethylene terephthalate) (PET) and PET based copolyesters (e.g. PETG). Crown Copyright (c) 2005 Published by Elsevier Ltd. All rights reserved.