A major problem in the oil and gas industry is corrosion in sour (or H2S-containing) environments. Low concentrations of H2S act to inhibit corrosion, because of the formation of a protective iron sulfide film/scale on steel surfaces; large concentrations can lead to pitting, severe localized corrosion, and catastrophic failure. Therefore, the solubility and hydrothermal stability of iron sulfides is particularly relevant to corrosion. To understand the mechanisms that are associated with the hydrothermal formation of iron sulfides, it is important to reliably estimate equilibrium constants for all the reactions involved, including those for the dissolution and dissociation of H2S. In this study, equations for determining the equilibrium constants that are related to the solubility of mackinawite are evaluated, and the most appropriate expressions are described. Also, the most relevant expressions for the solubility of H2S and for the first dissociation constant of H2S are reviewed. It is recommended that the second dissociation constant be avoided in calculations, because of the large uncertainty involved in previously reported values.