This book reports the preparation and characterization of solution-cast composite membranes and nanofiber reinforced membranes obtained by electrospining technique. Analysis techniques of FTIR, DMTA, permeability and conductivity characterization suggest that the reinforced fibers enhanced the mechanical properties and produces a remarkable reduction of the methanol permeability. The conductivity of the hybrid membranes (thickness ≈ 50 µm) equilibrated with water, measured at 80º C by impedance spectroscopy, is practically similar to nafion membranes of the same thickness. The reinforced membranes are build with nanofibers of polyvinyl alcohol (PVA) and polybuthyl alcohol PBA have been prepared. These nanocomposite membranes were obtained by infiltrating Nafion? or other polymers such is polyether-ether-ketone sulfonated (sPEEK) into a porous mat made by electrospinning of an aqueous solution of PVA or PBA. The mat was composed of nanofibers with diameters between 100-250 nm, which were functionalized on their external surface with sulfonic acid groups. The performance of fuel cells containing different membrane-electrode assemblies (MEAs) for PEMFC application is studied and the kinetic parameters governing the voltage dependence on current density estimated. An analysis of the methanol permeability in composite materials was also investigated and compared with pristine Nafion in order to study the behavior for direct methanol fuel cell (DMFC) applications. Finally the DMFC performances of MEAs prepared from Nafion/PVA and pristine Nafion membranes were tested at 45, 70 and 95 ºC under various methanol concentrations, i.e., 1, 2 and 3 M. On the other hand, the performance of the MEAs measured in a single cell operating with H2/Air was also investigated.