The photo-oxidation of polypropylene/montmorillonite nanocomposites has been studied under irradiation at long wavelengths (lambda > 300 nm) at 60°C and in the presence of oxygen. The chemical modifications resulting from photo-oxidation were followed by IR and UVvisible spectroscopies for each component of these nanocomposites polypropylene (PP), organomodified montmorillonite (MMt), and the compatibilizing agent (maleic anhydride-graftedpolypropylene). We have found that the mechanism of photo-oxidation of the polypropylene component was not modified in the polymer-nanocomposite, but that the rates of oxidation were modified, leading to an unexpected decrease of the durability of the material.