Poly(styrene-co-acrylonitrile) (SAN) nanocompositeswere successfully made by melt compounding and exhibited improved thermal stability and reduced flammability. The organoclays used in these nanocomposites were based upon fluorinated synthetic mica (FSM) or montmorillonite (MMT). Four different organic treatments were used on the clay surface to study the effect of organic treatment on clay dispersion: dimethyl, bis(hydrogenated tallow) ammonium (DMHTA), methyl tallow bis-2-hydroxyethyl ammonium (MTBHA), triphenyl, n-hexadecyl phosphonium (TPHDP), and 1,2-dimethyl-3-n-hexadecyl imidazolium (DMHDI). Along with studying the effect of clay organic treatment on the nanocomposite formation and flammability, the effect of acrylonitrile content in the SAN on nanocomposite formation and flammability was also studied. The overall findings suggest that dispersion of clay into SAN is rather facile, but certain combinations of organic treatment and clay type resulted in microcomposites rather than nanocomposites. Flammability of these materials was measured by pulse-combustion flow calorimetry (PCFC), also known as micro-cone calorimetry. (C) 2004 Elsevier Ltd. All rights reserved.