A series of hybrid Pd-Nafion membranes were prepared with different immersing time in PdCl2 solutions followed by reduction in NaBH4 solutions. Their microstructures and chemical composition were characterized by wide angle X-ray diffraction spectroscopy (WXRD), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and scanning transmission electron microscopy (STEM). The hybrid Pd-Nafion membranes exhibited excellent inhibition in methanol crossover while maintaining high proton conductivity. Such a high performance membrane was interpreted by the different roles played in the presences of metallic Pd nanoparticles, Pd hydrides and Pd oxides in the membrane. Metallic Pd nanoparticles catalyzed the proton transport in the Nafion molecular structure. Possible existence of Pd hydrides was beneficial to proton transport, while the suggested formation of Pd-multilayered three dimensional structures within the ionic cluster networks was favorable in elimination of methanol crossover. (C) 2011 Elsevier B.V. All rights reserved.