Abstract: The hydrothermal sulfidization process, which has attracted attention in recent years, involves the disproportionation of elemental sulfur in aqueous solution. The kinetics of this disproportionation reaction was carried out. The effect of cupric ions in aqueous solution on this reaction was also investigated. The reaction was carried out in an autoclave. It was shown that elemental sulfur disproportionated in aqueous solutions of pH 6-10 according to the following equation: Experimental results can be correlated by a model of diffusion of elemental sulfur from sulfur droplets to the aqueous solution followed by a pseudo-first-order disproportionation reaction in the aqueous phase. The rate expression has the following form: -ln(1-α)=kt·t where kt = aCs, a is the interfacial area of 1 g of liquid sulfur in water, k1 is the pseudo-first-order reaction rate constant, Cs and D1 are solubility and diffusion coefficient respectively of sulfur in the aqueous phase, α is the fraction of conversion of elemental sulfur, and t is the time of reaction. The effect of pH on reaction rate was studied at 130℃. It was shown that the pseudo-first-order rate constant k1 is directly proportional to the concentration of OH- in the pH range of 6-7.5. The temperature dependence of k, was also studied at pH 6.3 and the apparent activation energy derived was 24.0 kcal/mol.