In situ X-ray absorption spectroscopy was used to provide the redox states of the NaI/I2 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 I2. 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 I2 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−/I2 redox couple proceeds via the formation of I 3 − ions as an intermediate step.