Operando spectroscopy has been applied during electrochemical redox reactions of Prussian blue within an aqueous KNO 3 electrolyte, and definitively shows that K + is the predominant species that balances structural charge during the relevant redox reaction. Within a highly crystalline Prussian blue sample, the intercalation process was reproducible over repeated charge/discharge cycles, although full deintercalation of K + was kinetically hindered. with residual evolution of K + species being observed by XANES at potentials beyond that of the observed current response.