Poly{2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride−1,3-phenylendiamine} copolymer (ULTEM) was subjected to thermoaging in an attempt to determine the structure of the species formed during oxidative degradation. The oxidative process was followed as a function of exposure time by using MALDI−TOF MS. Thermal oxidation produces charring after only 15 min and the formation of insoluble residue amounts to 50% after 180 min at 350°C. Highly valuable structural information (including the end groups) was extracted from the MALDI spectra of the thermally oxidized ULTEM soluble samples. Oxidized specimens contained acetophenone, phenylacetic acid, phenols, benzoic acid, bisphenol A, phthalimide, and phthalic anhydride end groups. The mechanisms accounting for their formation involve several reactions:  (i) cleavage of the diphenyl ether units; (ii) oxidative degradation of the …