The decomposition of pure polyethylene terephthalate (PET) and PET/silica nanocomposites was investigated by thermal gravimetry (TG) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The influence of the nanosized silica on the pyrolysis properties of the composites was found from the results that the activation energies of decomposition and the residual carbon content increase with silica nanoparticles. It is deduced that the increase of the activation energies and the residual carbon content result from the adsorption of the decomposed products on the surface of silica. (c) 2006 Wiley Periodicals, Inc.