A nanocomposite electrolyte comprising a polymer-salt complex (PEO25-NaClO4) and an organically modified sodium montmorillonite (Na-MMT) has been prepared using tape casting technique. The effect of clay concentration on structural, microstructural and electrical properties has been investigated. X-ray diffraction (XRD) analysis confirmed the formation and structure of the nanocomposite. FTIR analysis suggested a clear evidence of polymer-ion-clay interaction indicated by the changes appearing in the nu(ClO4-) and nu(C-O-C) bands as a function of the clay concentration. The surface properties and microstructure has been studied using atomic force microscopy (AFM) technique. Complex impedance spectrum (CIS) shows the existence of bulk and grain boundary effects in the nanocomposite films. The temperature dependence of electrical conductivity obeys VTF pattern of variation. (c) 2006 Elsevier B.V. All rights reserved.