The Nation membrane is a critical component in redox flow batteries, which are widely investigated for grid-scale energy storage. However, there is very limited understanding of the fundamental properties of the membrane materials in the working environment relevant to redox flow batteries. This paper presents results of the analysis of the Nafion-117 membrane used in a vanadium redox flow battery by X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR) spectroscopy, and ultraviolet/visible spectroscopy. The XPS study reveals the chemical identity and environment of vanadium cations accumulated at the surface. On the other hand, the O-17 NMR measurement explores the nature of the diffused vanadium cations inside the bulk part of the Nation and shows the chemical bonding of cations and the host membrane. The F-19 NMR shows that the basic Nafion structure is not altered by the vanadium cations diffused inside. Based on these spectroscopic studies, the chemical identity and environment of the diffused vanadium cations in the Nafion membrane are discussed. This study reveals important information on the origin of performance degradation of the membrane materials in vanadium redox flow batteries and provides clues on how to improve the chemistry and properties of the energy storage devices. (C) 2010 Elsevier B.V. All rights reserved.