A zinc-bromine redox flow battery (ZBB) has attracted increasing attention as a potential energy-storage system because of its cost-effectiveness and high energy density. However, its aqueous zinc bromide phase and non-aqueous polybromide phase are inhomogeneously mixed in the positive electrolyte. Furthermore, various equilibrium reactions, e.g., charge-transfer reactions, polybromide formation, and complexation, simultaneously occur in the battery. Because of these complex reactions, it is difficult to systematically analyze its electrolyte, which a component crucial for the stable operation of the battery. Especially, although the state-of-charge (SoC) of an electrolyte is crucial for preventing overcharging or discharging and side reactions, its accurate estimation is difficult. As a result, there have been few studies on estimation of the SoC in ZBBs. In this study, in situ Raman spectroscopy is employed for the real-time estimation of the SoC in 25 charge-discharge cycles. To exclude errors arising from the inhomogeneous dispersion of the non-aqueous phase, SoC is monitored on the negative electrolyte. External standard solutions are measured, and the calibration curve is constructed just before in situ measurements at every cycle to minimize instrumental errors, e.g., those caused by alignment. This in situ methodology exhibits high accuracy and reproducibility. (C) 2017 The Electrochemical Society. All rights reserved.