The simultaneous solubilities of the minerals pyrite, pyrrhotite, magnetite, sphalerite, galena, gold, stibnite, bismuthinite, argentite, and molybdenite have been measured in 0 to 5 m NaCl solutions from 200 degrees to 350 degrees C at CO 2 partial pressures from 0.69 to 172 bars with oxygen and sulfur fugacities controlled by the pyrite-pyrrhotite-magnetite buffer. The relative solubilities follow the sequence Sb > Fe > Zn > Pb > Ag, Mo > Au, Bi in chloride-free solutions, and Fe > Sb > Zn > Pb > Au > Ag,Mo,Bi in concentrated chloride solutions. Gold solubilities as high as 5 ppm were measured in 5 m NaCl solutions. The aqueous species responsible for ore transport have been proposed based on the dependence of the solubilities on NaCl concentration and previous work on individual metal systems described in the literature. Although iron, zinc, lead, gold, and silver form chloride or hydroxy-chlo-ride complexes in 0.5 to 5.0 m NaCl solutions under these conditions, molybdenite solubility is independent of chloride concentration and stibnite solubility shows an inverse relationship to chloride content. Molybdenum is probably transported as an oxyacid and antimony as a neutral hydroxide complex. Bismuth may be chloride complexed, but the solubility of bismuthinite is so low and the results so scattered that the hydrothermal speciation of this metal remains in doubt. In the chloride-free solutions, iron is present mainly as the simple ferrous ion; zinc, antimony, and bismuth as hydroxy complexes; lead as a carbonate complex; molybdenum as molybdic acid; and gold and silver as bisulfide complexes. Mixed metal complexes appear to be unimportant under the experimental conditions. The speciation results are consistent with the Pearson hard-soft rule.