In situ X-ray absorption spectroscopy was used to provide the redox states of the NaI/I-2 mediators in an all-solid-state mediator-enhanced supercapacitor during the charge/discharge processes. Results show that in the charge process from -0.08 (open-circuit voltage (OCV)) to 0.80 V at constant current of 5 mA, all of the I- ions were oxidized to I-2. However, in the discharge process from +0.80 V to OCV at constant current of -5 mA, only I-3(-) ions were detected indicating that the 12 molecules were reduced to I-3(-) ions. Upon a negative polarization from OCV to -0.80 V at constant current of -5 mA, the I-3(-) ions were reduced to I- ions. Based on combined in situ XAS and cyclic voltammetry data, we show that the I-/I-2 redox couple proceeds via the formation of I-3(-) ions as an intermediate step. Published by Elsevier B.V.