In the present work, syndiotactic polystyrene (sPS) based nanocomposites containing 3 wt% of multiwalled carbon nanotubes (MWCNTs) and nanodiamonds (NDs) were prepared using the melt-mixing technique. Transmission electron microscopy (TEM) micrographs verified a good dispersion of the nanoparticles in the polymeric matrix, which resulted in a slight improvement of the mechanical properties of the nanocomposites. The addition of MWCNTs promotes the formation of beta-type crystals while in sPS and sPS/NDs alpha-type crystals are mainly formed. From the thermogravimetric analysis experiments a substantial enhancement of sPS thermal stability was found, caused by the addition of nanofillers, especially from NDs. The temperature corresponding to a mass loss of 0.5% was found 295 degrees C for sPS, 352.4 degrees C for sPS-MWCNTs and 370.7 C for sPS-NDs nanocomposites. The analytical method of Py-GC/MS was also used for further characterization of the samples. The main chain fragments collected from sPS pyrolysis were styrene monomers, dimers and trimers, while the three pyrolization temperatures applied (390, 420 and 500 degrees C) indicated some variations in intensity of decomposition fragments. The decomposition temperature as well as the nanoparticle addition (NDs or MWCNTs) affected the intensity of the recorded fragments but did not induce a change on the decomposition mechanism of sPS. (C) 2014 Elsevier B.V. All rights reserved.