The Fourier transform infrared (FTIR) spectrum of polyethylene terephthalate (PET) fibers extracted from vascular prostheses was used to illustrate the mechanisms of structure degradation. A quantitative model for fitting infrared spectra of PET samples over the region from 3200 to 3600 cm(-1) has been evaluated. Examination of a series spectrum of PET vascular prostheses collected after aging showed significant chemical differences in comparison with the virgin prosthesis. Analysis of these spectrums showed an increase of the end group concentrations for the explants. We used a complementary technique, the chemical titration method, in order to validate (FTIR) results. A considerable loss of macromolecular weight was observed, which is related to the formation of the smaller molecules that may proceed by random scission. Even if the time clearly influences the level of degradation, the results highlight other mechanisms of degradation that are probably related to human metabolism.