By combined potentiometric, electron paramagnetic resonance (EPR), and visible/near-infrared spectroscopic measurements, the coordination of SO42- to V(IV) and V(V) in M2S2O7-M2SO4-V2O5 (M = K and Cs) melts, respectively, under SO2(g) and O-2(g) atmospheres at 450-470 degrees C has been investigated. The results for both systems are in accordance with the oxo sulfate vanadate equilibria VO(SO4)(2)(2-) + SO42- reversible arrow VO(SO4)(3)(4-) for V(IV), (VO)(2)O(SO4)(4)(4-) + 2SO(4)(2-) reversible arrow 2VO(2)(SO4)(2)(3-) + S2O72- for V(V), and 2VO(2)(SO4)(2)(3-) + SO2 + SO42- reversible arrow 2VO(SO4)(3)(4-) for the V(V)-V(IV) redox reaction in melts saturated with sulfate. Constants for these equilibria have also been obtained, as well as characteristic EPR parameters and molar absorptivities for the complexes. In addition, the mole fractions of M2SO4 in the M2S2O7-M2SO4/SO2(g) systems saturated with M2SO4 were found to be 0.050 19 for M = K at 450 degrees C and 0.070 02 for M = Cs at 470 degrees C, respectively.