The suitability of our on-line NMR method was first demonstrated by measuring the background crossover of 2,6-DHAQ in a redox-flow battery without any applied voltage. Throughout the experiment, 1 H NMR spectra of the catholyte are continuously collected via a flow NMR tube; each spectrum collected represents an average of the system over 5 min. Sample flow can lead to signal suppression at higher flow rates, so care was taken to ensure that all the NMR spectra were quantitative for 2,6-DHAQ (see Tables , Figure -S4 and Methods S4-S6). The crossover of 2,6-DHAQ was detected by the appearance of the three characteristic aromatic proton resonances in the catholyte spectra, which increase in intensity over time (Figure ). Interestingly, the chemical shifts of these peaks also decrease over time, which can be explained by changes in intermolecular interactions between 2,6-DHAQ 2-ions at higher concentrations, and changes in bulk magnetism. By quantifying the signal intensity for 2,6-DHAQ 2-proton A (HA), we observed that the concentration of 2,6-DHAQ in the catholyte linearly increases over the duration of the experiment (Figure , Cell 1). The rate of increase in 2,6-DHAQ concentration was used to estimate a permeability (see Method S9 and Equation S1). Our measured 2,6-DHAQ permeability for Cell 1, (4.41 ± 0.01) x 10 -10 cm 2 s -1 , is within the range reported in literature, However, we note that the previously reported permeability values vary significantly, highlighting another challenge with using this metric to assist with membrane testing and design. Our findings demonstrate that on-line NMR spectroscopy can be used as an alternative in situ method for measuring real background permeabilities in redox-flow batteries. Encouraged by these initial results, we then applied this technique to an operating battery (Figure ). Two identical cells (Cell 2 and 3) were prepared and cycled between 0.6 V and 1.5 V at three different currents (50 mA, 25 mA and 10 mA), with rest periods in between. To identify any temporal effects, the order in which each current was applied was varied between Cell 2 and 3 (see Figure and). We note that both reduced and oxidised 2,6-DHAQ are now present in the anolyte side; nevertheless, due to the relative redox potentials, 2,6-DHAQ is only present in its oxidised form when it is detected by NMR on the catholyte side (see Figures and Methods S6). Initially, the 2,6-DHAQ build-up profile looks similar for each cell (Figure ); however, once battery charge is initiated after a 12 h rest period, a marked increase in crossover rate is observed in the operating cells (Cell 2 and 3). The crossover rates in Cell 2 and Cell 3 are no longer constant, and now vary depending on the charging protocol used for each cell, demonstrating that the electrochemistry of the system plays an important role in 2,6-DHAQ crossover.