A model is developed for the thermodynamic properties of pyrargyrite (Ag,Cu ) 3SbS 3 and high-skinnerite (Cu, Ag ) 3SbS 3 solid solutions over the temperature range 150-350°C. It makes explicit provision for AgCu exchange equilibria between pyrargyrite, high-skinnerite, and polybasite and for the presence of a miscibility gap between pyrargyrite and high-skinnerite. These features are constrained by 150-350°C AgCu exchange experiments (evacuated silica tubes; variable mass ratio) between pyrargyrite and polybasite and high-skinnerite and polybasite. AgCu exchange experiments between polybasite and equimolar mixtures of pyrargyrite and skinnerite are used to define the limbs of a miscibility gap between pyrargyrite and high-skinnerite. Explicit provision for site distortion resulting from the Cu + for Ag + substitution (or Ag + for Cu + substitution) is made by assuming that the standard state properties of the Ag-pyrargyrite and Cu-pyrargyrite endmembers are linearly dependent on the Cu/(Cu + Ag) ratio. Copper-substitution in pyrargyrite is shown to be asymmetric and substantially nonideal ( WCuAgPyr = 21.0; WAgCuPyr = 11.0 ± 0.5 kJ/gfw). In contrast, substitution of Ag in high-skinnerite is strongly ordered ( ∆d Gs∗ = 10 ± 0.5 kJ/gfw) on two sites (2:1) ( WCuAg ASkn = 12.0; WCuAg BSknA = 9.0 ± 0.5 kJ/gfw). Finally, these equilibria are consistent with estimates of Gibbs energies of formation (from the simple sulfides) of a hypothetical Cu 16Sb 2S 11 polybasite endmember of 15.00 kJ/gfw (400°C).