Spherical silica nanoparticles were mixed with a PP matrix and the thermal behavior of the nanocomposites was studied. The nanocomposites presented drastic improvements in the degradation behavior under thermo-oxidative conditions, showing complex multistep processes. Under inert conditions the improvements were lower. Our results indicate that mechanisms based on the labyrinth effect, nanoconfinement or trapping model, are not able to explain the whole enhanced thermal stability in these nanocomposites. Moreover, the high specific area of the nanoparticles (approximate to 70 m(2).g(-1)) indicates that processes based on the adsorption of volatile polar products coming from the oxidative degradation mechanism are plausible.