Thermal- and photo-oxidation processes occurring in poly(1,4-butylene terephthalate) (PBT) were investigated and compared in order to find out potential dissimilarities in the oxidation pathways of this polymer. The oxidized compounds were analyzed by matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). Applications of MALDI to the study of polymer photo- and thermo-oxidation are quite recent and involve the collection of MALDI spectra at different irradiation times and/or temperatures to observe the structural changes induced by heat or light under an oxidizing atmosphere. The polymer sample can be directly analyzed and the recorded MALDI spectrum arises from a mixture of non-oxidized and oxidized chains. The results obtained by MALDI are highly informative, as compared with previous studies based on conventional techniques, yielding precise information on the size, structure, and end groups of molecules derived from the oxidation process. According to the structure of the compounds identified in this Work, two thermal-oxidation Processes and two photo-oxidation cleavages have been unambiguously ascertained to occur in PBT. Furthermore, the inspection of the data acquired by MALDI allowed discriminating the degradation mechanisms entirely induced by light or heat without the involvement of oxygen. (C) 2008 Elsevier Ltd. All rights reserved.