Novel thermosetting poly[-oligo(ethylene glycol) diacrylate]-sodium montmorillonite nanocomposites containing a range of clay volume fractions were prepared by an in situ polymerization method. X-ray diffraction showed that the basal plane spacing of the clay was increased to approximately 1.7 mn regardless of clay volume fraction. Transmission electron microscopy confirmed the basal spacing and intercalated structure. The elastic moduli of the composites were measured using ultrasonic pulse-echo equipment. The results show that the Young's modulus and shear modulus increase with nominal clay volume fraction up to 0.22, and are in good agreement with the well-established Christensen method and derived Hashin-Shtrikman bounds for conventional composites provided that the true volume fraction of clay reinforcement filler is calculated. At low clay volume fractions, the composites were transparent. When the nominal clay volume fraction was further increased, cracks and porous surfaces appeared, as observed by scanning electron microscopy. These defects decreased the elastic modulus, indicating an upper limit for clay additions in this preparation route. (c) 2005 Wiley Periodicals, Inc.