Vanadium redox flow battery (VRFB) is currently the most mature technology of MW-level electricity storage. At present, the open circuit voltage of the electrolyte (OCV) is used to estimate the battery SOC (state-of-charge). Theoretically, the battery SOC cannot be estimated by measuring battery OCV along. Two analytical methods, UV-VIS absorption spectroscopy and redox electrode potential titration, were evaluated for assessing the electrolyte SOC. The UV-VIS spectra of solutions containing pure V²⁺, V³⁺, VO²⁺ ions can be identified. However, solutions containing a mixture of these ions cannot be easy identified. Redox potential titration analysis can identified individual ions from electrolyte sampled from positive electrode and negative electrode at SOC at 10%, 50%, 90%. Both OCV and UV-VIS are in-situ SOC monitoring method. The electrolyte composition is needed to be checked by redox potential titration at regular basis.