Electrospray ionization-mass spectrometry (ESI-MS) and proton nuclear magnetic resonance (1H NMR) are used to investigate the hydrolytic degradation of copolymers obtained by bulk ring-opening copolymerization of glycolide and ε-caprolactone with monomer ratios ranging from 70/30 to 30/70. NMR allows to follow changes of the average sequence distribution and composition of the components. In contrast, ESI-MS is susceptible to reveal the detailed chemical structures of various sequences despite the molecular weight limit of 2000 Da. Combination of ESI-MS with NMR can thus provide information to describe microstructure changes during degradation. The distribution of various oligomers shown in the form of planar projections is of great interest for the design of biodegradable system aimed at medical applications.