The V-V and V-IV oxosulfato complexes formed in V2O5-M2S2O7-M2SO4 (M = K, Cs) melts under SO2(g) or O-2(g) atmosphere have been studied by electronic absorption (VIS/NIR) and Raman spectroscopy at 450 degreesC. VIS/NIR spectra have been obtained at 450 degreesC for V2O5-K2S2O7 molten mixtures in SO2 atmosphere (P-SO2 = 0-1.2 atm). The data are in agreement with the V-V <----> V-IV equilibrium: (VO)(2)O(SO4)(4)(4-)(1) + SO2(g) <----> 2VO(SO4)(2)(2-)(1) + SO3(g). SO2 does not coordinate to the V-V complex but starts significantly to coordinate to V-IV for P-SO2 > 0.4 atm according to VO(SO4)(2)(2-)(1) + SO2(g) <----> VO(SO4)(2)SO22-(1). The Raman spectral features and the exploitation of the relative Raman intensities indicate that the (VO)(2)O(SO4)(4)(4-) dimeric complex unit, possessing a V-O-V bridge, is formed in the V2O5-M2S2O7 binary mixtures. The spectral changes occurring upon interaction of the binary V2O5-K2S2O7 mixtures with SO2 or addition Of M2SO4 to the binary V2O5-M2S2O7 mixtures indicate a cleavage of the V-O-V bridge and formation of the (VO)-O-IV(SO4)(2)(2-) Or (VO2)-O-V(SO4)(2)(3-) monomeric complex. units, respectively. The most characteristic bands due to the various complexes in the melts have been assigned. The spectral data are discussed in terms of possible structures. For the first time, high-temperature vibrational spectroscopy has been used to study the structural and vibrational properties of V2O5-K2S2O7 and V2O5-K2S2O7-K2SO4 melts. The results are valuable for the mechanistic understanding Of SO2 oxidation at the molecular level.