As the change in molecular weight of the starting material PCL and the obtained polyester was the smallest for the low molecular weight PCL (Mn = 5 kDa), we focused on analyzing the exact identity of these samples. Mass spectrometric investigations using matrix-assisted laser desorption ionization (MALDI) confirmed the findings observed by NMR spectroscopy and GPC. As the molecular mass of a HA units is the same as two CL units and both, tandem mass spectrometry experiments were performed using MALDI TOF-TOF MS to investigate the composition after the catalytic reaction. By comparison of the fragmentation mass spectrum (Fig. ) of the PCL starting material and the formed P(CL-HA)-copolymer for selected mass peaks (e.g. 1513 m/z) several additional ion peaks were identified in the P(CL-HA)copolymer (e.g. [C6H8O3+H] + at 129 m/z or [C12H18O4+H] + at 227 m/z), which clearly indicate the presence of adipic acid ester units together with CL units.