The ultraviolet-visible spectra of catholytes for vanadium flow batteries (VFBs) were measured and analyzed for a range of V-IV:V-V ratios and vanadium concentrations. Using a model of V2O33+ in equilibrium with VO2+ and VO2+, the spectra were characterized in terms of an excess absorbance parameter p and the molar extinction coefficients epsilon(4) and epsilon(5) of VO2+ and VO2+, respectively. The results showed that p varies weakly with the vanadium concentration C and this variation was quantified relative to a reference concentration C-r by means of a concentration coefficient phi(r). Experimental data showed that plots of fr versus C phi(r) and plots of 1/phi(r) versus C are linear and, based on this linearity, phi(r) was expressed as a simple function of C in terms of its reference concentration C-r and a single parameter M that is independent of the choice of C-r. Standard spectra of p at a concentration C-0 = 1 mol dm(-3) and of epsilon(4) and epsilon(5) were generated from which the spectrum of any catholyte may be simulated using the measured value of M in a governing equation. This enables determination of the state of charge for any VFB catholyte using absorbance measurements at a small number of wavelengths. (C) The Author(s) 2015. Published by ECS. All rights reserved.