Ternary phases in the system Cu-Sb-S are famatinite (Cu 3 SbS 4 ), chalcostibite (CuSbS 2 ), tetrahedrite and a compound unknown as a mineral, with the ideal formula Cu 3 SbS 3 . The only compound with an extensive composition field is tetrahedrite, for which the composition Cu (sub 12+x) Sb (sub 4+y) S 13 is proposed, where 0 < or = x < or = 1.92 and -0.02 < or = y < or = 0.27, analogous to the tenhantite formula proposed by Maske and Skinner (1971). The wide composition field for tetrahedrite at high temperatures cannot be quenched to room temperature because of the appearance of a low temperatures solvus. The phase Cu 3 SbS 3 is formed by reaction of chalcostibite + tetrahedrite + antimony at 359 degrees C and it remains stable to its congruent melting point of 607.5 degrees C. Although the compound can readily be quenched to room temperature, slow cooling causes it to break down below 359 degrees C. The occurrence of Cu 3 SbS 3 as a mineral species seems doubtful.Phase relations are dominated by a join between chalcocite and antimony that is stable from 300 degrees C to the liquidus and is apparently stable down to room temperature. The join precludes the stable coexistence of any ternary compound with the copperantimony alloy compounds, accounting for their extreme rarity in nature.