Nanocomposites of blends of polymethylmethacrylate (PMMA) and poly (styrene-co-acrylonitrile) (SAN) with natural and organically modified montmorillonite clays (Cloisite (R) 25A and Cloisite (R) 15A) were prepared by melt mixing in a twin-screw extruder and the effect of clay on the phase separation morphology and physical properties of nanocomposites was investigated. Multi-pass samples were; those extruded once (one-pass), twice (two-pass) and three times (three-pass). Dispersion of clays in the matrix polymers was investigated using XRD and TEM. Interestingly enough, the clays were observed to be mainly located at the boundaries of PMMA and SAN for most of the nanocomposites. As the number of pass increased, the phase-separated domain size became larger for nanocomposites of PMMA/SAN containing PM, while nanocomposites with clay 25A or 15A showed less degree of growth in domain size in the TEM pictures. Viscosities of the continuous phase and separated domains, and the compatibilizing effect of clays were discussed as the probable explanations for these observations. These were supported by the rheological properties measurements, where the nanocomposites with clay 25A or 15A showed the higher complex viscosities than those of PM and also showed some shear thinning behavior. DSC and TGA analyses were also conducted. (c) 2006 Elsevier Ltd. All rights reserved.