In this work we present a redox-flow battery set-up, into which two reference electrodes were inserted. This system allows for the investigation of individual overpotentials in each half-cell. Further, polarization (E/i) curves were recorded for different states of charge (SOC) of the battery. The performance of pristine and thermally oxidized carbon felts was compared at the negative and positive side of the battery. From the reference electrode measurements we were able to attribute the performance gain of the oxidized felt to the overpotential at the negative half-cell. In contrast the positive side does not seem to improve significantly.