The effects of the dispersion and concentration of single walled carbon nanotube (SWNT) on the flammability of polymer/SWNT nanocomposites were investigated. The polymer matrix was poly (methyl methacrylate) (PMMA) and the SWNT were dispersed using a phase separation ('coagulation') method. Dispersion of SWNTs in these nanocomposites was characterized by optical microscopy on a micrometer scale. Flammability properties were measured with a cone calorimeter in air and a gasification device in a nitrogen atmosphere. In the case where the nanotubes were relatively well-dispersed, a nanotube containing network structured layer was formed without any major cracks or openings during the burning tests and covered the entire sample surface of the nanocomposite. However, nanocomposites having a poor nanotube dispersion or a low concentration of the nanotubes (0.2% by mass or less) formed numerous black discrete islands with vigorous bubbling occurring between these islands. Importantly, the peak heat release rate of the nanocomposite that formed the network layer is about a half of those, which formed the discrete islands. It is proposed that the formation of the discrete islands is due to localized accumulation of the nanotubes as a result of fluid convection accompanying bubble formation and rise of the bubbles to the surface through the molten sample layer and bursting of the bubbles at the surface. The network layer acts as a heat shield to slow the thermal degradation of PMMA. (C) 2004 Elsevier Ltd. All rights reserved.