The behavior of sulfur in silicate melts has been the focus of hundreds of studies that were conducted to constrain the relationships among various oxidation states of sulfur ( \({\text{S}}^{2 - },{\text{S}}^{4 + },{\text{S}}^{6 + }\) ) in silicate melts as a function of pressure, temperature, redox conditions, dissolved volatiles, the composition of the associated vapor, as well as additional solid/liquid sulfur-bearing phases. Early experiments by researchers in the steel industry revealed that the concentration of sulfur in a silicate melt depends on the FeO content of the melt. These experiments demonstrated that at constant pressure and temperature, the concentration of sulfur dissolved in aluminosilicate melt changes systematically with changing oxygen fugacity, \(f{\text{O}}_{2}\) , when the FeO content of the melt is held constant. Subsequent studies of natural silicate melts quenched to silicate glasses during volcanic eruptions confirmed the positive relationship between the abundances of sulfur and Fe dissolved in natural silicate melts over a wide range of compositions from basaltic to rhyolitic. The effects of other cations dissolved in the silicate melt on the behavior of sulfur in the melt have been explored by conducting experiments over a wide range of silicate melt compositions known to exist in nature. Careful measurement of the X-ray peak position of sulfur in silicate glasses determined by electron probe microanalysis (EPMA) revealed that silicate melts commonly contain both \({\text{S}}^{2 - }\) and \({\text{S}}^{6 + }\) and that relative abundances of these two sulfur oxidation states systematically changes with relative \(f{\text{O}}_{2}\) . Sulfide, \({\text{S}}^{2 - }\) , dominates the sulfur budget of silicate melts under reducing conditions whereas sulfate, \({\text{S}}^{6 + }\) , dominates under oxidizing conditions. Synchrotron X-ray absorption near edge structure (XANES) analysis of quenched silicate melts provides the most robust understanding of the relationship between \(f{\text{O}}_{2}\) and the relative ratio of sulfate to sulfide, commonly expressed as \({\text{S}}^{6 + } /\sum {\text{S}}\) . The presence of additional oxidation states of sulfur including \({\text{S}}^{1 - }\) , \({\text{S}}^{0}\) and \({\text{S}}^{4 + }\) has been proposed by some studies, but these sulfur species are subordinate to \({\text{S}}^{2 - }\) and \({\text{S}}^{6 + }\) . The relationship between \(f{\text{O}}_{2}\) and \({\text{S}}^{6 + } /\sum{\text{S}}\) has significant implications for the mobility of sulfur in silicate magmas that contain an exsolved magmatic-hydrothermal C–O–H–S fluid. Experimental studies have demonstrated that \(f{\text{O}}_{2}\) , as it does for silicate melts, controls the ratio of reduced to oxidized sulfur in magmatic-hydrothermal fluids that dominantly contain \({\text{S}}^{2 - }\) as H2S under reducing conditions and sulfite, \({\text{S}}^{4 + }\) as SO2, under oxidizing conditions. In oxidized magmas, the reduction of sulfur from \({\text{S}}^{6 + }\) dissolved in silicate melt to \({\text{S}}^{4 + }\) in exsolved magmatic-hydrothermal fluid results in the oxidation of \({\text{Fe}}^{2 + }\) to \({\text{Fe}}^{3 + }\) in the melt. This behavior in turn has important consequences for the mobility of metals such as Au, which forms strong complexes with reduced \({\text{S}}^{2 - }\) in the magmatic-hydrothermal fluid, relative to metals such as Cu that does not complex with \({\text{S}}^{2 - }\) in the magmatic-hydrothermal fluid, and for the mobility of chalcophile metals dissolved in sulfide liquid that can be destabilized and resorbed during the mass transfer of sulfur from melt to fluid in oxidized magma.