Two classes of hybrid inorganic-organic proton-conducting membranes consisting of Nafion and either [(ZrO2)center dot(HfO2)(0.25)] or [(SiO2)center dot(HfO2)(0.28)] nanofiller are investigated to elucidate their relaxations and conductivity mechanism and are labeled [Nafion/(ZrHf)(x)] and [Nafion/(SiHf)(x)], respectively. The membranes are studied by dynamic mechanic analysis (DMA) and broadband electric spectroscopy (BES). The latter technique allows a determination of the direct current ionic conductivity (sigma(DC)) and the proton diffusion coefficient (DH+). Pulse-field-gradient spin-echo nuclear magnetic resonance experiments (PFGSE-NMR) are carried out to determine the water self-diffusion coefficients (D-H2O). DH+ and D-H2O are correlated to obtain insight on the conductivity mechanism of the proposed materials. Results indicate that the nanofiller particles play a major role in the proton conduction mechanism of the proposed materials. It is demonstrated that the basic [(ZrO2)center dot(HfO2)(0.25)] nanoparticles form Nafion-nanofiller dynamic cross-links with high ionic character. These cross-links improve the mechanical properties and enhance the overall proton conductivity of the membranes at low humidification levels owing to an efficient delocalization of the protons. In [Nafion/(SiHf)(x)] membranes, the dynamic cross-links occur due to dipole-dipole interactions between the side groups of the Nafion host polymer and the quasi-neutral [(SiO2)center dot(HfO2)(0.28)] nanoparticles. These cross-links significantly reduce the delocalization of the protons, which decreases the overall conductivity of materials. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.