Sulfur is a key player in the formation of metal sulfide deposits on Earth. Quantitative knowledge of sulfur speciation in the fluid phase is thus necessary to understanding sulfur degassing from magma and its transfer by the fluid phase across, together with metals, across the lithosphere. God gifted sulfur with an extraordinary aqueous chemistry, with a plethora of intermediate-valence species, including radical ions, polysulfides, sulfur dioxide and sulfites, coexisting with the two major redox end-members, sulfide S(-II) and sulfate S(+VI), all of them exhibiting various protonation and ion pairing reactions in aqueous solution. Most of those species are unquenchable, undergoing strong transformations upon fluid cooling thereby making their preservation difficult (if not impossible) in ex-situ studies. Quantifying such fugitive species requires in-situ spectroscopic approaches applied to the fluid and melt phases complemented by thermodynamic modeling. However, in-situ studies are rare and existing thermodynamic datasets for almost all S forms including sulfate and sulfide remain poorly constrained. As a result, they suffer from large uncertainties when extrapolating the species thermodynamic properties to temperatures and pressures beyond those covered by the limited experimental measurements upon which the models have been parameterized. We will show a few spectacular examples of such discrepancies when estimating sulfur speciation in fluids both in high-pressure subduction-zone and lower-pressure magmatic-hydrothermal contexts. The only way towards reducing these discrepancies is to extend the temperature and pressure range covered by experimental measurements by developing complementary spectroscopic approaches. We will show a few advances in constraining sulfur speciation using Raman and synchrotron X-ray spectroscopies such as sulfur L2,3-edge NRIXS and K-edge XANES. These and other promising in-situ spectroscopic methods should in the future improve speciation predictions and discriminate among different inconsistent and controversial thermodynamic models for sulfur and associated metals in geological fluids.