High-impact polystyrene (HIPS)/organically modified montmorillonite (organoclay) nanocomposites were synthesized via in situ polymerization. The effects of the organoclay on the morphology and material properties of HIPS/organoclay nanocomposites were investigated. Xray diffraction and transmission electron microscopy experiments revealed that intercalation of polymer chains into silicate layers was achieved, and the addition of nanoclay led to an increase in the size of the rubber domain in the composites. In comparison with neat HIPS, the HIPS/organoclay nanocomposites exhibited improved thermal stability as well as an increase in both the complex viscosity and storage modulus. The presence of intercalated organoclay drastically enhanced the gas-barrier properties because of the increase in the tortuosity of the diffusive path for a penetrating gas molecule. Some mechanical properties, including the tensile modulus, were superior to those of conventional HIPS. Finally, the preparation of the nanocomposites with a minimal loss of impact properties was proposed through changes in the synthetic procedure. (C) 2008 Wiley Periodicals, Inc.