This article attempts to develop and prove a technique to determine the degradation of polyethylene terephthalate (PET) for vascular prostheses. The implicit goal is to be able to quantify the amount of degradation to study the effect of in vivo aging. Nuclear magnetic resonance spectroscopy (H-1-NMR) provides a comprehensive view of chemical macromolecular structures. Examination of a series of PET vascular prostheses showed significant chemical differences between the virgin prostheses and the explants collected after aging, especially for diethylene glycol and cyclic oligomers groups, Aging was investigated in term's of chemical scission of ester and ether linkages caused by hydrolytic reaction during the in vivo stay. Besides, we extended this H-1-NMR technique to determine hydroxyl end-group concentrations And therefore the average number of macromolecular weight. To validate H-1-NMR results, complementary techniques, the chemical titration method and the classical viscosimetric method, were used. The results showed an increase of hydroxyl end-group concentration and a decrease in the macromolecular weight for the explants. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2813-2825, 2009