Polyoxymethylene (POM)/organo-modified bentonite nanocomposites are successfully prepared by melt intercalation method of which a primary ammonium salt is an effective surfactant, as evidenced from improvement in mechanical and gas barrier properties. Nanocomposite structures analyzed by XRD and TEM show mixed nanostructure of flocculation and exfoliation (flocculated/exfoliated nanocomposite) when primary ammonium-treated bentonite is used, whereas the quaternary ammonium-treated bentonites induce the mixture of intercalation and flocculation (intercalated/flocculated nanocomposite). The incorporation of organo-modified bentonite gives an effect on crystallization by generating numerous nucleating sites, especially in the case of bentonite with primary ammonium surfactant. The nanocomposites obtained exhibit improvement in flexural strength, flexural modulus, and elongation at break. The thermal degradation temperature is decreased by 40 degrees C' whereas the oxygen barrier is increased by 50%, as compared to neat POM. (c) 2008 Elsevier Ltd. All rights reserved.