Abstract Iron–sulfur (Fe-S) clusters are ubiquitous as redox-active protein cofactors, but it is often difficult to collect protein structures in which redox centres are in uniform and well-defined oxidation states. Using spinach ferredoxin I (Fdx) as a model redox protein, we demonstrate an integrated methodological pathway for electrochemical modulation of redox state in protein crystals coupled with in crystallo EPR and online-UV-visible spectroscopy to verify oxidation state. We show that Fd