Polyesters exhibit different end-group compositions depending on how they have been synthesized. The mass spectra obtained from the original polymer sample PES2_NC (control experiment for PES2 with no enzyme in the media (Fig. ) features three different end-group types-i.e., di-acid, mono-acid, di-alcoholas well as cyclic chains (lacking end-group contribution in their mass). The cyclic, di-and mono-acid terminated chains are the most abundant signals in the spectra of the starting material making 85% of the sum of the monoisotopic peaks (51% being cyclics, 34% mono-acid, 14% di-acid and 1% di-alcohol terminated). This results only partially agrees with NMR data where acid end-groups accounted for ca. 14 mol% over the MW range (Table ). Because of known ionization yields differences of end groups in ESI-MS, it not possible to comment on the relative abundance of oligomers as a function of their chain-end in the same sample. However, it is possible to assess differences in their relative contributions between samples. In particular, the MS signature of the degraded PES2_LCC sample (Fig. ) differs significantly from the MS profile of the original polymer. Here signals assigned to di-alcohol-terminated chains are predominant, with 88% of the sum of the monoisotopic peak intensities. The secondary most abundant signals were assigned to cyclic oligomers, while peaks assigned to mono-and di-acid terminated chains were in the minority, with intensities of 10% and 2% maximum, respectively. Experiments by changing the ionization additive from sodium to ammonium revealed similar end-group changes.