Reversed brackets on As and Sb partitioning between tetrahedrite-tennantite fahlores {∼Cu₁₀(Fe,Zn)₂(Sb,As)₄S₁₃} and bournonite-seligmannite solid solutions {CuPb(Sb,As)S₃} (400°C, evacuated silica tubes, NH₄Cl flux) indicate that the maximum nonidealities associated with the As-Sb substitution are about 165 (bournonite) and 250 (fahlore) cal/gfw on a per atom exchange basis. The experimental constraints are consistent with the following calibration of the As-Sb exchange reaction between these phases: RTln [ (1− X3FAH ) XAsBRN X3FAH XSbBRN ] = Δ G¯ As(Sb) −1 °FB + 14 ( Δ G¯ 23 ° ( X2 ) + WAsSbFAH ( 1 − 2 X3 ) ) − WAsSbBRN ( 1 − 2 XAsBRN where Δ G¯ As(Sb) −1 °FB { ˜ Δ H¯ As(Sb) −1 ° } = − 1.39 ± 0.10, WAsSbFAH = 4.00 ± 0.80, and WAsSbBRN = 0.66 ± 0.18, X₂≡Zn/(Zn + Fe) (or one-half the number of Zn atoms/formula unit) and X₃≡As/(As + Sb) refer to atomic ratios in fahlore, and Δ G¯ 23 ° = Δ H¯ 23 ° = 2.59 ± 0.14 kcal/gfw (Raabe and Sack, 1984; Sack and Loucks, 1975; O'Leary and Sack, 1987; Sack, 1992).