Vinylester resin-clay hybrids were prepared by the mixing different types of organically-modified montmorillonite (OMMT) with vinylester resin (VER) prepolymer, followed by thermal polymerization. VER prepolymer was synthesized from the reaction of diglycidylether of bisphenol-A (DGEBA) with acrylic acid. Various types of organic ammonium salts have been used as intercalating agents for montmorillonite, including N,N-dimethyl-N-(4-vinylbenzyl)stearyl ammonium chloride (VSA), N-allyl-N,N-dimethyl-stearyl ammonium chloride (ASA) and N,N-dimethyl-stearyl ammonium chloride (SA). The dispersion of OMMT into VER matrix was studied by XRD, which indicates the dependence of the morphology mainly on the OMMT content. UV-vis spectra of the hybrids were used to give a quantitative value of the effect of OMMT content on the transparency of VER/OMMT hybrid films. Also, the Vickers test has been performed to study the effect of OMMT content on the surface hardness of the hybrid films. In addition, the thermal properties of the hybrids have been characterized by measuring the softening points and thermogravimetric analyses of the hybrids in comparison with the pure resin. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 2060-2068, 2010