Thermodynamic calculations of the systems Te-O-H, Au-Te-Cl-S-O-H, and Ag-Te-Cl-S-O-H up to 300 degrees C, using the isocoulombic principle, show that the nature of aqueous tellurium species in equilibrium with native tellurium and the nature of aqueous chloride and sulfide species in equilibrium with calaverite (AuTe 2 ) and hessite (Ag 2 Te) are strongly dependent upon physicochemical conditions.Te 2 (super 2-) is the most important aqueous tellurium species in equilibrium with native tellurium in epithermal or mesothermal deposits in which sericitic alteration is dominant, and f (sub O 2 ) is near the hematite-magnetite buffer. HTeO 3 (super -) is the dominant aqueous species at f (sub O 2 ) conditions more oxidizing than the hematite-magnetite buffer, whereas HTe (super -) is dominant under reduced conditions. For Au and Ag contents of between 0.1 and 1 ppb, Sigma S = 0.01 m, m (sub Cl (super -) ) = 1 (conditions typical of epithermal or mesothermal gold-silver ore-forming fluids), and a Sigma Te = 1 ppb, calaverite is precipitated generally from solutions enriched in Au(HS) 2 (super -) and HAu(HS) 20 , whereas hessite is precipitated from solutions enriched in Ag(HS) 2 (super -) at near-neutral to slightly alkaline conditions, and from AgCl 2 (super -) at f (sub O 2 ) conditions higher than the hematite-magnetite buffer. The stabilities of binary silver tellurides and ternary gold-silver tellurides cannot be calculated due to a lack of thermodynamic data.The stability of calaverite can be used, in conjunction with silicate-sulfide-oxide assemblages, to determine the physicochemical conditions of gold-silver telluride formation. For example, the mutual stability of calaverite, quartz, pyrite, hematite, chalcopyrite, bornite, and sericite at 200 degrees C for conditions of Sigma S = 0.001 m, m Ca (super 2+) = 0.019, m Na (super +) = 0.25, m Mg (super 2+) = 0.002, and m K (super +) = 0.02 suggests that stage 1b mineralization in the upper portions of the Golden Sunlight deposit, Montana, precipitated from a fluid with an f (sub O 2 ) between 0.10 (super -40) and 10 (super -37) and a pH of 4.3 to 5.7. The stability of hessite covers most conditions of ore formation (oxidizing to reducing, and acid to alkaline) and is a less useful indicator of ore-forming conditions.