In this report, we will describe the effect of different concentrations of HfO2 nanopowders on the structure and properties of [Nafion/(HfO2)(n)] membranes with n = 0, 3, 5, 9, 11, 13, and 15 wt %, respectively. Films were prepared by a solvent casting procedure using HfO2 oxoclusters and Nafion. Seven new homogeneous membranes were obtained with thicknesses ranging from 200 to 350 mu m. Each membrane is characterized by a rough HfO2-rich surface and a smooth Nafion-rich surface, with different physical-chemical properties. Membrane characterization was accomplished by means of thermogravimetric analysis (TGA), morphological measurements (environmental scanning electron microscopy) and vibrational spectroscopy (Fourier transform infrared attenuated total reflectance spectroscopy and Fourier transform Raman spectroscopy). These systems can be described in terms of five types of water domains, Nafion-HfO2 species with well-defined stoichiometry surrounded by Nafion and hydrated hafnia. The highest conductivity at 125 degrees C (3.2 x 10(-2) S.cm(-1)) was measured on the [Nafion/(HfO2)(5)] film by electrical spectroscopy, with a stability range of conductivity between 5 and 115 degrees C.