Sulfur isotope exchange between sulfide (H 2 S) and thiosulfate (HSSO 3 H) can be described by the general rate law for a two-compound system (X and AB) with three exchangeable atoms (X, A, and B) proposed by [X. Chu, H. Ohmoto, Kinetics of isotope exchange reactions involving intra- and intermolecular reactions: I. Rate law for a system with two chemical compounds and three exchangeable atoms. Geochim. Cosmochim. Acta 55 1991 1953–1961]. According to the rate law, the isotope exchange reaction is comprised of one overall intramolecular exchange between sulfane (–SH or SH) and sulfonate (–SO 3 H or SO 3 H) sulfurs of thiosulfate (i.e., SH⇔SO 3 H in thiosulfate) and two overall intermolecular exchanges between sulfide and sulfane sulfur of thiosulfate (i.e., H 2 S⇔SH of thiosulfate) and between sulfide and sulfonate sulfur of thiosulfate (i.e., H 2 S⇔SO 3 H of thiosulfate). The rate constants for the three overall exchange reactions and the equilibrium isotopic fractionation factors among sulfide, sulfane, and sulfonate of thiosulfate were estimated by fitting [F. Uyama, H. Chiba, M. Kusakabe, H. Sakai, Sulfur isotope exchange reaction in the aqueous system: thiosulfate–sulfide–sulfate at hydrothermal temperature. Geochem. J. 19 1985 301–315] experimental data on sulfur isotope exchange between aqueous H 2 S and sodium thiosulfate by the least squares method. At temperatures of 100–170 °C, the equilibrium fractionation factors (in per mil) can be expressed as:1000lnαH2S–SH=−0.327±0.055(1012/T4)+2.676±0.341(106/T2)1000lnαSO3H–SH=−0.352±0.009(1012/T4)+7.523±0.054(106/T2)and1000lnαSO3H–H2S=−0.0293±0.058(1012/T4)−4.871±0.357(106/T2)(T in K). At near-neutral pH, the overall rate (m −1 s −1 ) for the sulfur isotope exchange between H 2 S and –SO 3 H of thiosulfate is described bylogkSO3H⇔H2S=−5.14(103/T)+10.35(T in K) with an activation energy of 98.3 kJ/mol at 100–170 °C.A comparison of the rates of sulfur exchanges among H 2 S, –SH, and –SO 3 H of thiosulfate with the rates of polysulfide–thiosulfate formation and disproportion reactions determined by [W.F. Giggenbach, Kinetics of the polysulfide–thiosulfate disproportionation up to 240 °C. Inorg. Chem. 13 1974b 1730–1733] suggests that the sulfur isotope exchanges between aqueous sulfide and thiosulfate may proceed via the formation and disproportionation of polysulfides (e.g., S 3 S 2− , S 4 S 2− , etc.):10H 2 S+3S 2 O 3 2− =4S 3 S 2− +2H + +9H 2 OandS n S 2− +SSO 3 2− =S n+1 S 2− +SO 3 2− . The disproportionation reaction of polysulfides appears to control the exchange rate between S 2− and S 6+ atoms in thiosulfate and is considered the rate-determining step in the sulfate–sulfide exchange reaction rather than the intramolecular exchange of thiosulfate proposed by [H. Ohmoto, A.C. Lasaga, Kinetics of reactions between aqueous sulfates and sulfides in hydrothermal systems. Geochim. Cosmochim. Acta 46 1982 1727–1745]. Therefore, polysulfides may play an important role in the chemical and isotopic reactions between aqueous sulfide and sulfate under hydrothermal conditions.