The structural and vibrational properties of Nb-V oxosulfato complexes formed in Nb2O5-K2S2O7 and Nb2O5-K2S2O7-K2SO4 molten mixtures with 0 <= X-Nb2O5(0) <= 0.25 have been studied by high-temperature Raman spectroscopy under static equilibtrium at temperatures up to 700 degrees C. The spectral features for the binary Nb2O5-K2S2O7 molten system indicate that the dissolution of Nb2O5 proceeds with consumption of S2O72- leading to the formation of a Nb-V oxosulfato complex according to Nb2O5 + nS(2)O(7)(2-) -> C2n-; a simple formalism exploiting the relative Raman band intensities is used for determining the stoichiometric coefficient, n, pointing to n = 3 and to the following reaction: Nb2O5 + 3S(2)O(7)(2-) -> 2NbO(SO4)(3)(3-), which is consistent with the Raman spectra of the molten mixtures. Nb2O5 could be dissolved much easier when K2SO4 was present in an equimolar (1:1) SO42-/Nb ratio; the incremental presence of K2SO4 in Nb2O5-K2S2O7 melts induces composition effects in the Raman spectra that terminate when n(SO42-)/n(Nb) = 1. The composition effects and the temperature-dependent features of the Raman spectra obtained for Nb2O5-K2S2O7-K2SO4 molten mixtures together with the spectral changes occurring upon freezing are accounted for by a Nb2O5 center dot 3K(2)S(2)O(7)center dot 2K(2)SO(4) stoichiometry for the complete reaction taking place: Nb2O5 + 3S(2)O(7)(2-) + 2SO(4)(2-) -> NbO(SO4)(4)S2O77- + NbO2(SO4)(2)(3-). The spectral data are discussed in terms of the most plausible structural models, for which consistent band assignments are made. The most characteristic Raman bands for the Nb-V oxosulfato complexes pertain to Nb=O modes: (i) at 937 cm(-1) for the mono-oxo Nb=O mode of NbO(SO4)(3)(3-); (ii) at 958 cm(-1) for the mono-oxo Nb=O mode of NbO(SO4)(4)S2O77-; and (iii) at 926 cm(-1) for the symmetric dioxo Nb(=O)(2) mode of NbO2(SO4)(2)(3-).