In this paper, we characterize the absorbance spectra and explain the spectroscopic behavior typically found in VRFB electrolytes at various mixture ratios of VII/VIII and VIV/VV mixtures in H2SO4. We demonstrate that the linear behavior of absorbance with respect to mixture ratio for the anolyte (V II/VIII) can be used to determine mixture ratio. In addition, we find that the non-linear behavior of the absorbance with respect to mixture ratio for the catholyte (VIV/VV) is due to the formation of a vanadium complex.