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 degrees C in a LW 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, plithalic anhydride, and plithalic acid end groups. The mechanisms accounting for the formation of photooxidation products of Ultem involve several reactions: (i) photocleavage of methyl groups of the N-methyl phthalimide terminal units; (ii) photooxidative degradation of the isopropylidene bridge of BPA units; (iii) photooxidation of plithalimide units to phthalic anhydride and plithalic acid end groups. Some of these cleavage pathways are specific for the photooxidation process and the oligomers deriving from them are absent in the thermal by oxidized Ultem samples, whereas the thermal cleavage of the diphenyl ether units appears to occur only in the thermal oxidation process.