Organoclay nanocomposites were prepared by ultrasound-assisted solution intercalation technique based on polystyrene containing brominated epoxy and a combination of brominated epoxy and antimony oxide. Aspects of nanomorphology and nanodispersion were investigated by X-ray diffraction and transmission electron microscopy whereas flammability and reaction to fire were evaluated using limiting oxygen index, UL-94, and mass loss calorimeter tests. Polystyrene/brominated-epoxy-blend-based nanocomposites showed mixed intercalatedexfoliated nanomorphology where polymer-intercalated crystallites predominantly exist in polystyrene matrix and exfoliated silicate layers reside on polystyrene/brominated epoxy phase boundaries and within brominated epoxy domains. Organoclay was found to impart a compatibilization effect on polystyrene and dispersed brominated epoxy, which facilitates uniform distribution of a fine flame-retarding phase within the matrix. With the reduction of the rate at which decomposition products evolve into the gas phase, organoclay nanocomposites showed notable reductions in peak heat release rate and increases in limiting oxygen index. The gas-phase hot radical entrapment by halogenated flame-retardant system was coupled with the condensed-phase physical action of nanodispersed organoclay, which increased the overall fire-retardant effectiveness. Fire-retardant mechanisms of nanocomposites based on polystyrene/brominated epoxy blends were attributed to nanoconfinement and tortuous pathway effects of organoclay rather than to carbonaceous char formation proposed earlier for polystyrene/organoclay systems without conventional flame retardants. Copyright (C) 2011 John Wiley & Sons, Ltd.