The photodegradation of EPDM based on dicyclopentadiene was followed by FTIR spectroscopy and the main photo-oxidation products were identified by derivatization reactions. It could be found that the photodegradation is initiated at the diene with formation of alpha,beta-unsaturated carbonyl compounds and is then propagated to the ethylene-propylene segments. The propagation reactions are accelerated and, hence, the rate of photodegradation increases with the diene content.