Determination of phase relations in the Ag-Au-S system between 100 degrees and 850 degrees C by silica tube, differential thermal analysis, and high-temperature X-ray experiments reveals that the condensed system consists of three basic parts: the Ag-Au complete solid solution, the Ag 2 S-Au 2 S partial solid solution, and sulfur. At low temperatures, two ordered silver-gold sulfides exist, Ag 3 AuS 2 and AgAuS; they invert at 185 degrees and 310 degrees C, respectively, to a simple, cubic solid solution. Above 310 degrees C a complete solid solution exists from Ag 2 S to approximately Ag (sub 0.9) Au (sub 1.1) S. This solid solution has a body-centered cubic structure for silver-rich compositions, which changes via a second-order transition to a simple, cubic structure between Ag 2 S and Ag (sub 1.5) Au (sub 0.5) S. The silver-rich end of the solid solution inverts to a face-centered modification at high temperatures. The sulfides melt in the interval 680 degrees to 838 degrees C with a minimum in the liquidus at Ag (sub 1.3) Au (sub 0.7) S.Enthalpies of transition for the Ag 2 S III-II, Ag 3 AuS 2 low-high, and AgAuS low-high transitions were measured by differential scanning calorimetry and found to be, respectively: 950; 2,270(min)-2,760(max); and 1,570(min)-1,920(max) calories/mole. Within experimental error, the sulfide solid can be modeled as a regular solution with components AgS (sub 0.5) and AuS (sub 0.5) and with W G [asymp] - 1,000 calories. Estimated temperature-dependent standard free energies of formation are (T = K and units of calories): -43,880 + 20.8T (acanthite, Ag 2 S III), -40,410 + 20.8T (uytenbogaardtite, low Ag 3 AuS 2 ), -37,910 + 15.3T (high Ag 3 AuS 2 ), -36,740 + 23.4T (low AgAuS), -33,340 + 17.6T (high AgAuS), and -22,700+ 29.6T (Au 2 S).