This work has been focused on the characterization of the methanol permeability and fuel cell performance of composite Nafion/PVA membranes in function of their thickness, which ranged from 19 to 97 mu m. The composite membranes were made up of Nafion (R) polymer deposited between polyvinyl alcohol (PVA) nanofibers. The resistance to methanol permeation of the Nafion/PVA membranes shows a linear variation with the thickness. The separation between apparent and true permeability permits to give an estimated value of 4.0 x 10(-7) cm(2) s(-1) for the intrinsic or true permeability of the bulk phase at the composite membranes. The incorporation of PVA nanofibers causes a remarkable reduction of one order of magnitude in the methanol permeability as compared with pristine Nafion (R) membranes. The DMFC performances of membrane-electrode assemblies prepared from Nafion/PVA and pristine Nafion (R) membranes were tested at 45,70 and 95 degrees C under various methanol concentrations, i.e.,1, 2 and 3M. The nanocomposite membranes with thicknesses of 19 mu m and 47 mu m reached power densities of 211 mW cm(-2) and 184 mW cm(-2) at 95 degrees C and 2M methanol concentration. These results are comparable to those found for Nafion (R) membranes with similar thickness at the same conditions, which were 210 mW cm(-2) and 204 mW cm(-2) respectively. Due to the lower amount of Nafion (R) polymer present within the composite membranes, it is suggested a high degree of utilization of Nafion (R) as proton conductive material within the Nafion/PVA membranes, and therefore, significant savings in the consumed amount of Nafion (R) are potentially able to be achieved. In addition, the reinforcement effect caused by the PVA nanofibers offers the possibility of preparing membranes with very low thickness and good mechanical properties, while on the other hand, pristine Naf (R) ion membranes are unpractical below a thickness of 50 mu m. (C) 2010 Elsevier B.V. All rights reserved.