Nanocomposite formation brings about an enhancement of many properties for a polymer, including enhanced fire retardancy. This study was carried out to determine if the presence of clay causes changes in the degradation pathway of polystyrene. In the case of virgin PS, the degradation pathway is chain scission followed by beta-scission (depolymerization), producing styrene monomer, dimer and trimer, through an intra-chain reaction. As the clay loading is increased, the evolved products produced through inter-chain reactions become significant. Due to the barrier effect of the clay layers, the radicals have more opportunity to undergo radical transfer, producing tertiary radicals, and then radical recombination reactions, producing head-to-head structures, and hydrogen abstraction from the condensed phase also occurs. In the presence of clay, the color of solid residues darkens as the clay content increases. It is thought that this color change is caused by the formation of conjugated double bonds in the presence of clay. (c) 2005 Elsevier Ltd. All rights reserved.