The photo-oxidation behavior under natural and accelerated conditions of polypropylene/layered silicate nanocomposite is studied in this article. The nanocomposites are prepared via simple melt mixing (extrusion and injection molding). The structure obtained is very dependent on the preparation mode and the modified clay used; mostly, exfoliation structure is produced. The nanocomposites start their photo-degradation earlier than the control samples polypropylene and polypropylene-graft-maleic anhydride with a higher oxidation rate for specimen produced by injection molding. This is explained by the presence of organiphilic-modified montmorillonite layers that trap the oxygen, increasing the oxygen pressure in the bulk and leading to a decrease of the induction period. Contrary to the control samples that display auto acceleration in their oxidation kinetics, the nanocomposites show a slight tendency to a plateau indicating a slowing clown of the photo-oxidation process. This is ascribed to oxygen starvation that occurs in the nanocomposite. The acceleration factor is found to be higher for the nanocomposite comparatively of the control samples. With the aid of SF4 and NO treatments, the mechanism of photo degradation was found to be similar in PPgMA and its nanocomposites. (c) 2007 Wiley Periodicals, Inc.