A polypropylene matrix was melt compounded with a given amount (2 vol %) of both untreated (hydrophilic) and surface treated (hydrophobic) fumed silica nanoparticles with the aim to investigate the influence of the time under processing conditions on the microstructure and thermo-mechanical properties of the resulting materials. Chain scission reactions induced by thermal processing caused a remarkable decrease of the melt viscosity, as revealed by the melt flow index values of both neat matrix and nanocomposites, but the degradative effect was significantly hindered by the presence of silica nanoparticles. It was observed that the size of nanofiller aggregates noticeably decreased as the compounding time increased, especially when hydrophobic silica nanofiller was considered. While the melting temperature seemed to be unaffected by the processing time, a remarkable embrittlement of the samples was observed for prolonged compounding times. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40242.