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°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°C　and　100 MPa,　gradually　converting　into　Os(IV)-OH-Cl　and　Os(II)-Cl　complexes　over　450°C.　The　equilibrium　constant　(ln　K)　(K = [HCl]4 ⨯ [Cl−]2/[OsCl62−])　between　OsCl62−　and　water　molecule　is　determined　as　ln　K = (50.43 ± 4.633) − (54223 ± 2525.6)/T,　and　ΔrHmΘ　and　ΔrSmΘ　are　inferred　to　be　(450.8 ± 21.00)　kJ　·　mol−1　and　(419.3 ± 38.52) J　·　mol−1　·　K−1.　Furthermore,　the　formation　constant　(ln　β)　of　OsCl62−　exhibits　a　change　from　−0.097　to　−0.104　as　temperatures　increase　from　150　to　400°C,　while　the　change　values　in　standard　Gibbs　free　energy　(ΔrGmΘ)　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°C　and　pH ＜ 5),　or　relatively　high-temperature　and　acidic-neutral　fluids　(＞300°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.　©　2024.　American　Geophysical　Union.　All　Rights　Reserved.