The thermal aging of polypropylene (PP)/SiO2 nanocomposite films was carried out at 130 degrees C. In contrast to the widely accepted thermal oxidation mechanism, the film ruptured far before the carbonyl group was detected and without a noticeable reduction in the molecular weight. Observations with a polarizing optical microscopy and a scanning electron microscopy demonstrated that, instead of oxidative degradation, at least three other factors were responsible for the rapid deterioration of the PP/SiO2 nanocomposites: (1) recrystallization during the thermal aging, which gave rise to a major volume contraction and, thus, great stress along the spherulite boundaries, which might have induced cracks there (another effect of the recrystallization was the rejection of nanoparticles and defects to the spherulite boundaries, which, therefore, weakened the boundaries); (2) poor interfacial interaction between the matrix and the nanoparticles; (3) large temperature changes, which created interfacial debonding because of the significant difference in the thermal expansion coefficients of PP and the nanoparticles. The results of this study extend the understanding of the thermal oxidative degradation mechanism of polymer materials. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 601-606, 2009)