Compared with other methods of large-scale energy storage, organic redox flow batteries (ORFBs) have emerged as a promising technology class for reliable and cost-effective integration of renewables into the electricity grid. Operated by pumping solution-phase redox active species from external storage tanks into electrochemical cells, ORFBs enable independent scaling of energy and power densities using materials that are cheap, safe and earth-abundant. As for long discharge durations, ORFB levelized costs depend critically on electrolyte lifetime, the development of new or tailored techniques capable of rationalising full cell capacity-fade in terms of specific mechanisms of molecular loss is critical. This is especially true for long-cycling cells exhibiting capacity fade rates of less than 0.1% / day, for which methods based on conventional galvanostatic cycling have proven to be unreliable. Recently, two in situ NMR methods were developed that enabled real-time evaluation of electrolyte decomposition mechanisms and battery self-discharge. Here, we propose to use such methods, along with a variety of electrochemical techniques, to deconvolute simultaneous contributions to capacity-fade arising from active species crossover and electrolyte decomposition. Using a diverse synthetic library of viologen-based active species, which we will develop based on preliminary results obtained during the Midi project, we will investigate changes in chemical stability, solubility and trans-membrane flux that take place during cycling. Additionally, by oligomerising redox active species, complex processes involving inter-pendant electron transfers that influence electrochemical kinetics can also be studied. We ultimately hope to tie these trends to structural features of both the redox active molecules and membrane materials to enable design of new, ultra-long cycling ORFB systems.