Metal　complexation　and　speciation　is　the　primary　process　responsible　for　metal　transport　and　circulation　in　hydrothermal　systems,　during　which　stable　and　soluble　metal　complexes　play　a　pivotal　role.　Here,　we　investigate　the　speciation　of　Os　and　the　thermodynamic　stability　of　Os(IV)-Cl　complexes　in　chloride-bearing　solutions　at　temperatures　ranging　from　150　to　600　degrees　C　and　pressure　of　100　MPa　through　hydrolysis　experiments.　The　results　show　that　the　dominant　species　of　Os　is　OsCl62-　at　temperatures　between　150　and　450　degrees　C　and　100　MPa,　gradually　converting　into　Os(IV)-OH-Cl　and　Os(II)-Cl　complexes　over　450　degrees　C.　The　equilibrium　constant　(ln　K)　(K　=　[HCl](4)　x　[Cl-](2)/[OsCl62-])　between　OsCl62-　and　water　molecule　is　determined　as　ln　K　=　(50.43　+/-　4.633)　-　(54223　+/-　2525.6)/T,　and　Delta　H-r(m)Theta　and　Delta　S-r(m)Theta　are　inferred　to　be　(450.8　+/-　21.00)　kJ　center　dot　mol(-1)　and　(419.3　+/-　38.52)　J　center　dot　mol(-1)　center　dot　K-1.　Furthermore,　the　formation　constant　(ln　beta)　of　OsCl62-　exhibits　a　change　from　-0.097　to　-0.104　as　temperatures　increase　from　150　to　400　degrees　C,　while　the　change　values　in　standard　Gibbs　free　energy　(Delta(r)G(m)(Theta))　for　the　hydrolysis　reactions　decrease　with　rising　temperature,　suggesting　a　temperature-dependent　thermodynamic　stability　of　OsCl62-.　Geochemical　modeling　further　demonstrates　that　high　solubility　of　OsCl62-　could　exist　in　low-temperature　and　acidic　fluids　(＜=　300　degrees　C　and　pH　＜　5),　or　relatively　high-temperature　and　acidic-neutral　fluids　(＞300　degrees　C　and　pH　＜　7),　primarily　influenced　by　the　Cl　concentration.　Acidic　and　near-neutral　fluids　with　high　Cl　concentration　venting　in　the　mid-ocean　ridge,　back-arc,　and　sediment-hosted　systems　contribute　more　to　dissolving　and　transporting　Os　from　the　lithosphere　to　the　hydrosphere,　thereby　impacting　the　global　ocean　dissolved　Os　budget.　Plain　Language　Summary　The　rare　and　precious　metal　osmium　is　typically　enriched　in　the　deep　Earth　and　associated　mantle-derived　rocks.　However,　some　submarine　sediments　and　hydrothermal　fluids　may　contain　elevated　levels　of　Os,　suggesting　a　potential　pathway　for　Os　transport　from　the　lithosphere　to　the　hydrosphere.　The　key　point　is　to　clarify　in　what　form　and　how　Os　transports　from　the　deep　Earth　to　the　seawater.　To　achieve　this　goal,　hydrolysis　experiments　were　conducted　to　determine　the　speciation　and　stability　of　Os　in　chloride-bearing　solutions.　The　results　revealed　that　OsCl62-　can　remain　stable　in　low-temperature,　then　convert　into　Os(IV)-OH-Cl　and　Os(II)-Cl　complexes　over　450　degrees　C　at　similar　to　NNO　(Ni-NiO)　oxygen　fugacity　conditions.　Quantitative　relationship　among　Os　solubility,　chloride　concentration,　pH,　and　temperature　was　established,　indicating　that　high　Os　solubility　could　exist　in　the　low-temperature　and　acidic　fluids　(＜=　300　degrees　C　and　pH　＜　5)　or　relatively　high-temperature　and　acidic-neutral　fluids　(＞300　degrees　C　and　pH　＜　7),　such　as　those　found　in　submarine　vent　fluids　in　mid-ocean　ridge,　back-arc,　and　sediment-hosted　systems.　During　ascent,　these　Os-rich　hydrothermal　fluids　can　mix　with　seawater,　thereby　dominating　Os　transport　and　distribution　in　submarine　hydrothermal　systems.　This　study　provides　new　evidence　and　insights　into　Os　transport　and　enrichment　mechanisms　responsible　for　transporting　Os　from　the　lithosphere　to　the　hydrosphere.