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The reduced sulfur species, sulfide, elemental and thiosulfate were considered in the thermal waters of Jiangxi Province for the first time. It is shown that the sulfur speciation content significantly varies and depends on the pH values. The major part of reduced sulfur refers to sulfide species in the nitric thermal waters, to elemental—in the carbon dioxide thermal waters. The presence of both reduced and oxidized sulfur speciation indicates the possibility of sulfide minerals hydrolysis and disproportionation of the product of this reaction (SO2) with the participation of hot water with the formation of elemental and sulfate sulfur. The isotopic composition of dissolved sulfate and sulfide sulfur speciation has shown that the process of bacterial reduction proceeds in the thermal waters, accompanied by accumulation of relatively heavy sulfur isotope in sulfates. Simultaneously with reduction, the oxidation of both sulfide minerals and newly formed hydrosulfide proceeds with formation of elemental, thiosulfates and also sulfates in the discharge zone was proceeded. It is shown that the process of sulfide oxidation mostly occurs in carbon dioxide thermal waters. Therefore, the elemental sulfur is predominant in carbon dioxide waters. The oxidation process is less significant in the nitric thermal waters, whereby the concentrations of sulfide ion are higher than sulfates. The ambiguous effect of sulfate reduction on the hydrogeochemical environment of the thermal waters is confirmed by the differing value of the carbon isotope ratio of HCO3- in the considered waters. The obtained isotopic composition data 34dS(SO42-) indicate host rocks as a source of sulfates in the thermal waters of Jiangxi Province.