Nafion-tungsten oxide hybrid membranes, [Nafion/(WO3)(x)], with varying loading levels of WO3 nanofiller (x = 0, 0.024, 0.329) are prepared and investigated as candidates for application as solid electrolytes in vanadium redox flow batteries (VRFBs). The thermal properties of [Nafion/(WO3)(x)] hybrid membranes are probed both by high-resolution thermogravimetric analysis (HR-TGA) and by modulated differential scanning calorimetry (MDSC). Vibrational spectroscopy studies are carried out by: (i) Attenuated Total Reflectance - Fourier Transform Infrared spectroscopy (ATR-FTIR); and (ii) Raman spectroscopy, to elucidate the secondary structure of [Nafion/(WO3)(x)]J and study the interactions taking place between the nanofiller and the Nafion matrix. The electrical response of [Nafion/(WO3)(x)]J is determined by Broadband Electrical Spectroscopy (BES) and the permeability towards VO2+ is measured by UV-VIS spectrometry. It is demonstrated that the [Nafion/(WO3)(x)] hybrid membranes exhibit a high ion selectivity (up to 10.6.10(3) S.min-cm(-2) for [Nafion/(WO3)(0.329)]) that is much improved in comparison with that characterizing recast Nafion (6.5-10(3) S.min.cm(-3)). A structural model and a conductivity mechanism for the [Nafion/(WO3)(x)] hybrid membranes are proposed, in order to rationalize the experimental results and correlate the electrical response with the transport properties.