The photochemical behaviour of polypropylene–clay nanocomposites was studied. The samples were prepared by blending in the molten state isotactic polypropylene, organo-modified montmorillonite, i.e. layered silicate or clay modified with alkyl ammonium cations, and maleic anhydride-grafted polyprolylene as compatibilizer. The montmorillonite dispersion state within the iPP matrix was analyzed by Transmission Electron Microscopy. Accelerated photoageing of nanocomposites carried out at l>300 nm and 60°C was analysed by infrared spectroscopy. The oxidation products formed have been identified and the contribution of each component of the nanocomposite evaluated by studying different formulations. The presence of stabilisers in the formulation has also been analysed from the comparison of the rates of oxidation of several stabilised and non-stabilised nanocomposites.