Two preparation techniques attempting to disperse nanoclays in high-impact polystyrene matrix yielded different clay dispersion states either as intercalated or phase-separated morphologies. By this means, the influence of micro- and nanocomposite formation on the synergistic flame retardancy between nanoclays and a conventional mineral-type flame-retardant additive, namely aluminium tri-hydroxide, was investigated in terms of limiting oxygen index, horizontal burning rates and cone calorimetric fire properties. Reductions in peak heat release rates in the cone calorimeter were doubled with nanocomposites relative to microcomposites, attributed to char enhancement and lower mass loss rates. This was accompanied by higher limiting oxygen index, lower burning rates and better mechanical properties. In particular, the formation of nanocomposites allowed for the recovery of tensile strength reductions caused by high loadings of aluminium tri-hydroxide in the polymer. (C) 2010 Elsevier Ltd. All rights reserved.