The mechanism of photodegradation of poly(neopentyl isophthalate), an aromatic polyester as model for industrial polyester coatings, was studied on the molecular level. Changes in the chemical structure of molecules caused by UV irradiation (mercury lamp) were investigated using several analytical techniques. Photodegradation leads both to chain scission and to crosslinking, taking place simultaneously as measured by SEC. Extensive exposure results in appreciable amount of insoluble material (gel). Generation of carbonyl C=O and hydroxyl OH/OOH groups in the polymer structure was monitored with ATR-FTIR. MALDI-ToF MS provided detailed structural information on the degradation products of the polyester. In the initial stage of degradation Norrish photocleavage (type 1) takes place. Radicals generated in this reaction (photolysis) can directly abstract hydrogen or can react with oxygen creating primarily acid and hydroxyl end groups (photooxidation). Moreover hydrogen abstraction taking place along the polymer backbone followed by oxidation reactions leads to further fragmentation of the polymer chain. The highly informative data provided by MALDI-ToF MS allowed establishing the pathways of photolysis and photooxidation. (C) 2009 Elsevier Ltd. All rights reserved.