Abstract A detailed study on the in vitro degradation of a poly(ethylene glycol) and poly(butylene terephthalate) (PEOT/PBT) segmented block copolymer was carried out using liquid chromatography/electrospray–mass spectrometry. Accelerated hydrolysis of PEOT/PBT was achieved by placing the material for 14 days in a refluxing phosphate buffered saline (pH 7.4) solution. All major degradation products and several side-products were identified using both the positive and the negative ion mode. The data indicate that degradation does not only occur in the “soft”, but also in the “hard” segments of the polymer. Liquid chromatographic separation is required to distinguish between degradation products with different sequences but identical molecular mass. The addition of ammonium and sodium ions provided important complementary information on the number of monomer units present in the degradation products.