Fingerprinting of the degradation product patterns by electrospray ionization mass spectrometry (ESI-MS) was evaluated as a tool to monitor the degree of degradation in polyester-ether networks. Four different crosslinked caprolactone (CL) and/ or 1,5-dioxepan-2-one (DXO) networks were subjected to hydrolytic degradation in aqueous solution at 37 degrees C for up to 147 days. After predetermined time periods, the water-soluble degradation products were analyzed by ESI-MS and tandem ESI-MS. In addition, changes in pH, mass loss, and copolymer composition were determined. In the case of more slowly hydrolyzed CL-rich (co)polymers, CL and/or DXO oligomers terminated by hydroxyl and carboxyl end groups were predominantly formed as degradation products. However, on prolonged hydrolysis oligomers with attached crosslinking agent dominated the degradation product patterns of more easily hydrolyzed DXO-rich (co)polymers. It was shown that in the recorded mass spectra the variation of intensities in the series of ions corresponding to DXO and CL/DXO oligomers with or without attached crosslinking agent could be utilized to monitor the extent of hydrolytic degradation in the polyester matrix and the disruption of the network structure. (C) 2008 Wiley Periodicals, Inc.