We showed that it is easy, in principle, to measure the state of charge (SoC) of VRFB negative electrolytes by ultraviolet-visible (UV-Vis) absorption spectroscopy because the spectra are a linear combination of the spectra of the components, VII and VIII. In contrast we showed, for a range of mole percentages of VV at several different concentrations of total vanadium, that the UV-Vis spectra of the positive electrolyte do not exhibit a similar linearity and that absorbance measurements at two different wavelengths are required to measure the SoC. All positive electrolytes showed an excess absorbance Aex which we have proposed to be proportional to the concentration of a 1:1 mixed-valence complex V2O33+ in equilibrium with VO2+ and VO2+. We have defined a parameter p which directly relates Aex to the vanadium concentration C and the mole percentage of VV (i.e. the SoC). We showed that at any given wavelength, the value of p decreases somewhat with C. This is consistent with our model, when the variation of the V2O33+ equilibrium constant Kc with C is small. We estimated approximate values for Kc and for the excess molar extinction coefficient εex.