The sulfur compounds released by volcanic eruptions, generally believed to be in the form of SO2 and H2S, may cause global cooling of the atmosphere. However, several recent field and experimental studies suggested that under moderately oxidized conditions hexavalent sulfur species may coexist with SO2 in magmatic fluids and may later be directly emitted at volcanic vents, which contradicts some thermodynamic predictions. We have investigated sulfur speciation in magmatic-hydrothermal fluids by loading different amounts of dilute sulfuric acid into a hydrothermal diamond-anvil cell and performing in situ Raman spectroscopy at temperatures up to 700 °C. Upon heating SO42− disappeared beyond 100 °C, and SO2 formed at >250 °C probably due to reduction by the rhenium or iridium gasket. With high-fluid densities (such as >0.9 g/mL), the initial acid and air bubble homogenized into the liquid phase and most sulfur was present in the form of either HSO4− or H2SO4 (the rest being SO2) within investigated T–P conditions (with pressures up to 10 kb). With low-fluid densities (such as <0.2 g/mL), the system homogenized into the vapor phase and molecular H2SO4 appeared to dominate (with pressures less than 1 kb). These observations strongly suggest that hexavalent sulfur is stabilized by hydration in magmatic fluids.