The ion exchange of vanadium ions in different oxidation states existing as V2+ V3+,VO2+, VO with H3O+ across cation exchange membranes plays a certain role for the mechanism of energy loss in a vanadium redox flow battery. Ion exchange equilibria of these vanadium species with H3O+ ions have been measured with three kinds of cation exchange membranes using atomic absorption spectrometry as analytical tool. It turns out that a considerable amount of vanadium is soluble in the membranes which indicates that ion exchange with H3O+ ions can contribute to cross contamination in a vanadium redox flow battery and to corresponding energy losses.