The reactions of sulfur species (HS-, S, and S2O32-) under hydrothermal conditions are an important scientific subject due to their implications in biology and geology and potential application for hydrogen production as well. In this paper, effects of representative metals and alloy such as Ni, Fe, Co, W, Hastelloy C-276 alloy, SUS 316 alloy, and sulfide mineral Ni3S2 on reactions of sulfur species under alkaline hydrothermal conditions were studied. The results showed that there are mainly three types of reactions among sulfur species, H2O, and metal (or Ni3S2): (i) sulfidation of metals and Ni3S2 by sulfur species; (ii) disproportionation of sulfur species; (iii) oxidation of sulfur species by water to produce hydrogen. Co and Ni can be sulfided, and W remains unchanged in the presence of HS-. The presence of Ni and Ni3S2 would significantly enhance the transformation of S2O32- into SO32- and SO42-. This study would give clues to sulfur chemistry within hydrothermal waters and guide the selection of the reaction pathway of sulfur species by adding metals or Ni3S2.