A series of polymer-clay nanocomposite (PCN) materials that consisted of polystyrene-co-acrylonitrile) (PSAN) and layered montmorillonite (MMT) clay were successfully prepared by effectively dispersing the inorganic nanolayers of MMT clay into the organic PSAN matrix by a conventional in situ thermal polymerization. First of all, organic styrene and AN monomers at a specific feeding ratio were simultaneously intercalated into the interlayer regions of organophilic clay hosts and followed by a typical freeradical polymerization with benzyl peroxide as initiator. The as-synthesized PCN materials were subsequently characterized by FTIR spectroscopy, wide-angle powder X-ray diffraction, and transmission electron microscopy. The as-prepared PCN materials, in the form of coatings, incorporated with low clay loading (e.g., 1 wt %) on cold-rolled steel, were found to be much superior in corrosion protection over those of bulk PSAN based on a series of standard electrochemical measurements of corrosion potential, polarization resistance, and corrosion current in 5 wt % aqueous NaCl electrolyte. Molecular weights of PSAN extracted from PCN materials and bulk PSAN were determined by gel permeation chromatography with THF as eluant. Effects of the material composition on the molecular barrier and thermal stability of PSAN along with PCN materials, in the form of both membrane and fine powder, were also studied by molecular permeability analysis, differential scanning calorimetry, and thermogravimetric analysis, respectively. (C) 2004 Wiley Periodicals, Inc.