The thermodynamic relations between partial pressures and molecular or ionic activities of aqueous solutions containing S are presented graphically as functions of the 5 variables: T, P, pH, Po 2 , and (Sigma S) (the total activity of aqueous S species). These diagrams provide a quantitative basis for correlation of the composition of aqueous solutions to the fields of stability of minerals up to 250 degrees C. The correlation is based on the principle that at equilibrium, partial pressures fixed at any one temperature by a univariant assemblage of minerals in an anhydrous system are not affected by the addition of H 2 O if the assemblage is unchanged. The application of this principle to the anhydrous Fe-S-O system demonstrates that up to 250 degrees C. the common assemblage pyrite, pyrrhotite, and magnetite may be deposited at equilibrium only by alkaline ore solutions. This assemblage has been deposited in nature and is stable in the dry system over a temperature range from 675 degrees to below 250 degrees C. The association of sphalerite with pyrrhotite, pyrite, digenite, covellite, and barite, but not with free S, limits thermodynamically the aqueous conditions during deposition. At 250 degrees C. this ore solution must be neutral to weakly alkaline with HS (super -) as the predominant S-containing, aqueous ion; Ps 2 is usually between 10 (super -8.5) and 10 (super -14.7) atm. and Po 2 between 10 (super -36) and 10 (super -50) atm. Therefore, the solubility of sphalerite must be sufficient to account for the transport required for ore formation at 250 degrees C. under the conditions where pH=7.5, Po 2 =10 (super -40) and (Sigma S)=1.0 m.