This paper describes the effect of nanopowders acting as fillers on the structure and properties of [Nafion/(MxOy)(m)] membranes with n = 5 wt.%. The films were prepared by a solvent casting procedure using MxOy oxoclusters (M = Ti, Zr, Hf, Ta and W) and Nafion. Five new homogeneous membranes were prepared with thicknesses ranging from 170 to 350 mu m. The membranes were characterized by means of thermal analyses carried out by thermogravimetry (TG), morphological measurements by scanning electron microscopy (ESEM) and vibrational spectroscopy by FT-IR ATR, FT-Raman measurements. Vibrational investigations allowed us to obtain information on: (a) the network structure and the conformation of fluorocarbon hydrophobic domains of the Nafion host polymer, (b) the MxOy center dot center dot center dot HSO3-R interactions occurring in ionic polar cages of membranes and (c) the various water domains present in the hydrophilic polar cages of membranes. TG allowed us to determine that the membranes are thermally stable up to about 170 degrees C. In conclusion, it was disclosed that: (a) the hydrophobic Nafion polymerhost domains consist of a blend of PTFE chains with helical conformations 10(3) and 15(7) and (b) the fraction of each fluorocarbon helical conformation is modulated by the type of nano-fillers and the dynamic cross-links R-SO3H center dot center dot center dot MxOy center dot center dot center dot HSO3-R which occur in hydrophilic polar cages of membranes. In hydrophobic domains of Nation, the highest percentage of fluorocarbon chains with helical conformation 10(3) is observed for [Nafion/(HfO2)(n)] composite membrane, rho(%) = 16. (c) 2007 Elsevier Ltd. All rights reserved.