This work investigates the melt-processing of styrene-acrylonitrile copolymer (SAN) with organo-modified montmorillonite (MMT) clays and the influence of the clay on mechanical properties and on fire performance of SAN. The nanodispersion of MMT is evaluated qualitatively by X-ray diffraction (XRD) and transmission electronic microscopy (TEM), and quantitatively by solid state nuclear magnetic resonance (NMR). SAN/MMT nanocomposites reveal an intermediate morphology, an intercalated structure with some exfoliation and with the presence of small tactoids, whatever the loading in MMT is. The polymer-clay interfacial area is estimated at 0.5 and the degree of homogeneity characterizing the distribution of MMT platelets is about 40%. The presence of clay in SAN-25 leads to a 'filler effect' increasing the stiffness but decreasing tensile strength of the nanocomposites. It also leads to a significant decrease of peak of heat release rate measured by mass loss calorimetry. (C) 2004 Elsevier Ltd. All rights reserved.