The dissolution reaction of WO3 in pure molten K2S2O7 and in molten K2S2O7-K2SO4 mixtures is studied under static equilibrium conditions in the X-WO3(0) = 0-0.33 mol fraction range at temperatures up to 860 degrees C. High temperature Raman spectroscopy shows that the dissolution leads to formation of W-VI oxosulfato complexes, and the spectral features are adequate for inferring the structural and vibrational properties of the complexes formed. The band characteristics observed in the W=O stretching region (band wavenumbers, intensities, and polarization characteristics) are consistent with a dioxo W(=O)(2) configuration as a core unit within the oxosulfato complexes formed. A quantitative exploitation of the relative Raman intensities in the binary WO3-K2S2O7 system allows the determination of the stoichiometric coefficient, n, of the complex formation reaction WO3 + nS(2)O(7)(2-) -> C2n-. It is found that n = 1; therefore, the reaction WO3 + S2O72- -> WO2(SO4)(2)(2-) with six-fold W coordination is proposed as fully consistent with the observed Raman features. The effects of the incremental dissolution and presence of K2SO4 in WO3 K2S2O7 melts point to a WO3 center dot K2S2O7 center dot K2SO4 stoichiometry and a corresponding complex formation reaction in the ternary molten WO3-K2S2O7-K2SO4 system according to WO3 + S2O72- + SO42- -> WO2(SO4)(3)(4-). The coordination sphere of W in WO2(SO4)(2)(2-) (binary system) is completed with two oxide ligands and two chelating sulfate groups. A dimeric [{WO2(SO4)(2)}(2)(mu-SO4)(2)](8-) configuration is proposed for the W oxosulfato complex in the ternary system, generated from inversion symmetry of a WO2(SO4)(3)(4-) moiety resulting in two bridging sulfates. The most characteristic Raman bands for the W-VI oxosulfato complexes pertain to W(=O)(2) stretching modes (i) at 972 (polarized) and 937 (depolarized) cm(-1) for the v(s) and v(as) W(=O)(2) modes of WO2(SO4)(2)(2-), and (ii) at 933 (polarized) and 909 (depolarized) cm(-1) for the respective modes of [{WO2(SO4)(2)}(2)(mu-SO4)(2)](8-).