The UV-visible spectra of V-III-V-II electrolytes in 3 mol dm(-3) H2SO4 showed a linear relationship of absorbance to mole fraction but those of V-IV-V-V electrolytes did not. At all wavelengths, V-IV-V-V mixtures showed excess absorbance over that expected for a linear combination of the components. Plots of excess absorbance A(ex), against mole fraction f of V-IV were found to be symmetrical parabolas that reach a peak at a mole fraction f = 0.5. Excess absorbance is attributed to a strongly absorbing 1:1 mixed-valence complex, V2O33+, in equilibrium with VO2+ and VO2+. Based on this, an expression is derived that relates A(ex) to f and from this it is shown that when KcC < 1 (where K-c is the equilibrium constant and C is the total vanadium concentration), A(ex) should vary linearly with f(1 - f). Experimentally, plots of A(ex) versus f(1 - f) showed excellent linearity through the origin at all wavelengths, thus confirming the model. An excess molar extinction coefficient epsilon(ex) and an alternative excess absorbance parameter p are defined. Values of p and approximate values of K-c and epsilon(ex), were estimated; the spectrum of V2O33+ is plotted as its approximate molar extinction coefficient. (C) 2014 The Electrochemical Society. All rights reserved.