"Crossover" describes the unwanted transport of redox-active components through the membrane. This is generally assessed using single-electrolyte diffusion-cell experiments, though the measured permeabilities do not always correlate well with battery performance. In operating batteries, the oxidation states (and hence charge density) of redox-active species change and there are additional transport mechanisms to consider when a charge is applied to a cell (e.g. migration and electro-osmosis). Both the charge and size of the redox-active species have been shown to be important determiners of permeability rates through Nafion ion-exchange membranes, with charge being the dominant factor. However, these effects have not yet been studied in the battery environment, where there will likely be a trade-off between the increased membrane repulsion and increased migration as a result of a higher effective charge. To further advance membrane design, it is important that all the contributions to crossover in operating AORFBs are fully understood and measured. Though migration is a well-known phenomenon, its contribution to crossover has only been estimated based on electrochemical and theoretical studies. Here we report the first direct measurements of crossover in a fully operational AORFB.