The UV-light induced oxidation of ethylene-propylene-diene (EPDM)/montmorillonite nanocomposites obtained with synthetic montmorillonite was studied. The nanocomposites were obtained by melt compounding EPDM-g-MA (EPDM grafted with maleic anhydride as compatibilising agent) and organophilic synthetic clays. Four different samples were prepared and fully characterised: an EPDM grafted with maleic anhydride, an EPDM/Na-MMt microcomposite and two EPDM/MMt nanocomposites, one with intercalated MMt and the other with exfoliated MMt. The oxidation of the polymeric matrix was characterised by infrared and UV-visible spectroscopies. Special attention was given to the EPDM-g-MA sample in order to understand the complex modifications of the infrared spectra resulting from the presence of the compatibiliser. The photoproducts were the same in all samples. The formation of these oxidation products occurred after an induction period, explained by the presence of a residual processing antioxidant. The induction time was found to be reduced in the presence of MMt and the effect was enhanced in the case of the exfoliated nanocomposite. After the induction period, oxidation started with a rate that was independent of the presence of MMt.