Thermal oxidation and photooxidation processes occurring in poly 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride−1,3-phenylendiamine copolymer (ULTEM), were investigated and compared. The study aimed at finding possible differences in the oxidation pathways of this complex polymer by using the analytical power of MALDI techniques. ULTEM films were subjected to photooxidation by exposure at 60 °C in a UV accelerated chamber (Q-UV Panel) in atmospheric air, and the oxidative process was followed as a function of the exposure time. Relevant structural information on the photooxidized ULTEM species was extracted from the MALDI spectra. These data show the presence of polymer chains containing acetophenone, phenyl acetic acid, phenols, benzoic acid, phthalic anhydride, and phthalic acid end groups. The mechanisms accounting for the formation of photooxidation products of …