New nanocomposites based on a vinylester resin (VER) and the organo-montmorillonites (ODA-M, BHL-M, DEM-M, BHLV-M) modified with octadecylammonium, bis(2-hydroxyethyl)lauryl ammonium, diethyl[2-(methacryloyloxyl)ethyl]ammonium, and bis(2-hydroxyethyl)lauryl(vinylbenzyl) -ammonium chlorides, respectively, were prepared by dispersing the organoclays in VER, and subsequent crosslinking at finally 120 degreesC. X-ray diffraction studies and morphological studies using transmission electron microscopy revealed that exfoliation occurs for the VER/ODA-M and BHLM composites, intercalation occurs for the VER/BHLV-M composite, and neither intercalation nor exfoliation occur for the VFR/DEM-M and umnodified montmorillonite composites. On the whole, although the enhancement in flexural modulus was observed for the exfoliated ODA-M and BHL-M composites, the flexural strength was rather lowered. Dynamic viscoelastic measurement revealed that the ODA-M, BHL-M, and BHLV-M nanocomposites have significantly higher storage modulus at the rubbery state than the other composites. (C) 2004 Society of Plastics Engineers.