The influence of montmorillonite (MMT) silicate layers on the semicrystalline morphology development of the poly(ethylene oxide) (PEO) ,matrix of PEO/MMT nanocomposites has been investigated by using X-ray diffraction, differential scanning calorimetry, light microscopy and time-resolved simultaneous small- and wide-angle X-ray scattering. The silicate layers act as nucleating agents for the crystallization of PEO, but at high content, also have a retarding effect oil the crystal growth. In that case they are non-crystallisable barriers in the crystallization of the PEO matrix. The lamellar semicrystalline structures of pure PEO and the PEO/MMT nanocomposites, are, however, identical. (c) 2005 Elsevier Ltd. All rights reserved.