During the charging of a Zn/Br redox flow battery, cyclic voltammetry and electrochemical impedance spectroscopy measurements were carried out in-situ. As the state of charge (SOC) increased, some polybromide complex accumulated on the Br-side electrode surface and showed a positive effect on the adsorption of bromide ion as well as bromine. The deposition of polybromide complex onto the electrode surface was identified by SEM, EDS, and Raman spectroscopy. As a result, the charge transfer resistance for bromine oxidation decreased from 2.13 ohm to 1.27 ohm as the SOC increased from 0.0% to 80.0%. This may be due to the amphiphilic characteristics of the polybromide complex. While the solution resistance for catholyte was independent of the SOC, that for anolyte sharply decreased with increasing SOC. This could be explained by the increase in zinc ion mobility and the anolyte thickness reduction by growth of zinc metal dendrites.