The effects of surfactant structure on the morphology and mechanical properties of melt processed mixtures of poly(styrene-co-acrylonitrile) (SAN) with montmorillonite (MMT) organoclays were examined. The composite which exhibited the greatest change in gallery height, the highest modulus, and greatest aspect ratio (similar to 50) was produced from an organoclay with the lowest molecular weight surfactant, dimethyl hydrogenated tallow ammonium. For ammonium ion surfactants with similar to 18 carbon hydrophobic tail(s), stack swelling, as measured by wide angle X-ray scattering (WAXS), was more strongly related to the reduced surfactant molecular weight than polarity or aromaticity of the head group substituents. A surfactant with a shorter tail length (similar to 12 vs. 18 carbon tail length) resulted in unswollen stacks in the composite. The swelling of the organoclay galleries seems to be explained by a balance of some favorable interactions of SAN with the montmorillonite platelet surface, versus repulsive mixing of aliphatic surfactant substituents with polar SAN. The repulsive nature of the latter increases as molecular weight of the head group substituents increases. A third factor is the platelet-platelet attraction that seems to explain the lack of swelling in the case of the shorter tail surfactant. Mechanical properties are also reported; and the composite moduli were found to compare to theoretical predictions using the Halpin-Tsai theory based on aspect ratios determined by transmission electron microscopy (TEM). The experimental aspect ratios do not seem to correlate with the WAXS gallery shifts. This and other evidence suggest that exfoliation in melt-processed SAN/MMT composites is not explained by intercalation behavior. (c) 2005 Published by Elsevier Ltd.