The change in the degradation pathway of a polymer by incorporation of clay has a significant effect on the fire retardancy of polymer/clay nanocomposites. Since the clay layers act as a barrier to mass transport and lead to superheated conditions in the condensed phase, extensive random scission of the products formed by radical recombination is an additional degradation pathway of polymers in the presence of clay. The polymers that show good fire retardancy upon nanocomposite formation exhibit significant intermolecular reactions, such as inter-chain aminolysis/acidolysis, radical recombination and hydrogen abstraction. In the case of the polymers that degrade through a radical pathway, the relative stability of the radical is the most important factor for the prediction of the effect that nanocomposite formation has on the reduction in the peak heat release rate. The more stable is the radical produced by the polymer, the better is the fire retardancy, as measured by the reduction in the peak heat release rate, of the polymer/clay nanocomposite. (c) 2005 Elsevier Ltd. All rights reserved.