Sulfur occurs ubiquitously in crustal fluids as sulfate and sulfur dioxide (oxidised species) and as sulfide (the reduced species H2S and HS-). However, it has also long been known (Geitner, 1864; Lewis et al., 1918; Giggenbach, 1971) (but little studied) that other sulfur species such as the trisulfide ion, S31-, may also be stable in high temperature water. The trisulfide ion is unstable in ambient temperature water and the only “geological” evidence for its presence in the deep crust is manifested by the mineral lazurite, the main component of lapis lazuli. The intense blue colour has recently been shown (Fleet and Liu, 2010) to be due to trace amounts of the trisulfide ion chromophore, trapped in the lazurite structure during its formation. Recently, we have measured the Raman spectra of sulphur species in hydrothermal solutions up to 600°C and 2 GPa (20 kbar). The stability of various sulfur species in hydrothermal fluids in the Earth’s crust will be considered in terms of these data and recent literature studies. In addition, the results of recent experiments pertaining to the role of reduced sulfur in the transport and deposition chemistry of antimony in the Earth’s crust will be discussed.