We report experimental brackets (150–350°C, evacuated silica tubes) on (1) AgCu partitioning between hexagonally close packed (hcp)-, face centered cubic (fcc)-, and body centered cubic (bcc)-(Ag, Cu) 2S sulfides and the sulfosalts polybasite-pearceite [(Ag 1− x Cu x ) 16(Sb, As) 2S 11, 0.10 > x > 0.57] and high-skinnerite [(Cu 1− x , Ag x ) 3SbS 3, x < 0.26] and (2) Ag Cu ratios of (Ag, Cu 2S sulfides coexisting with polybasite and skinnerite, and polybasite and pyrargyrite [(Ag 1− x , Cu x ) 3SbS 3, x < 0.21]. Based on these brackets on AgCu partitioning and activity-composition models for polybasite-pearceite and high-skinnerite (Harlov and Sack, 1994, 1995) we infer that (1) both hcp- and bcc-(Ag, Cu) 2S sulfides exhibit positive deviations from ideality sufficient to produce metastable unmixing at slightly less than 90°C and (2) fcc-(Ag, Cu ) 2S sulfides display more moderate positive deviations from ideal activity-composition relations, characterized by a regular-solution-type parameter, W AgCu fcc = 5.4 ± 0.2 kJ/mol, for an assumed site multiplicity of AgCu mixing of 2. The inferred activity-composition relations for fcc- and bcc-(Ag, Cu) 2S are consistent with brackets on the T − X Cu systematics of the transition between these phases (Skinner, 1966) and the calorimetric data of Grønvold and Westrum (1986) for Ag 2S provided that the effective site multiplicity of AgCu mixing in bcc-(Ag, Cu) 2S decreases linearly from about 3 in Cu-poor to about 2 in Cu-rich compositions ( X Cu ∼ 0.315). Our constraints are also consistent with the calorimetric data for Cu 2S (Grønvold and Westrum, 1987) and Skinner's (1966) brackets on the T − X Cu systematics of the transition between fcc- and hcp-(Cu, Ag) 2S provided that the effective site multiplicity of AgCu mixing in hcp-(Ag, Cu) 2S approximates 3. From our brackets on Ag Cu ratios of (Ag, Cu) 2S sulfides coexisting with polybasite and skinnerite, and polybasite and pyrargyrite and calibrations for the AgCu exchange reactions we calculate average values of 5.30 ± 1.14 and −31.1 ± 0.7 kJ/mol for the Gibbs energies of formation of hypothetical Cu 16Sb 2S 11 and Ag 16Sb 2S 11 polybasite components from the simple sulfides over the temperature range 150–350°C.