There is an urgent need for new energy storage solutions that will support the decarbonization of the electricity grid. Aqueous organic redox flow batteries are low-cost, long-duration energy storage devices that are in the process of being commercialized for this application; however, their operational lifetime is limited by electrolyte decomposition and crossover. These degradation processes are generally studied separately, so the relationship between the two is poorly understood. Previously, it had been assumed that the main contribution to battery capacity fade was electrochemical degradation of the electrolytes. Using the on-line 1H NMR crossover characterization method we developed previously, we reveal the first evidence for crossover-driven side reactions in redox flow batteries. If the impact of these side reactions is not considered, it will lead to an underestimation of crossover and its impacts on battery lifetime. We further introduce simple ‘simulated-crossover’ experiments to identify anolyte-catholyte combinations where these processes are occurring. Using these simulated-crossover experiments, we find that crossover-driven side reactions can be mitigated by avoiding the use of anolytes with hydroxyl functional groups when using ferrocyanide electrolytes. These insights should be used to assist the design of new anolytes and catholytes, which will facilitate the development of longer-lasting redox flow batteries.