Phase relations in the system Cu-Co-S have been studied using sealed silica capsule methods between 400 degrees and 900 degrees C. The high-temperature portion of the system is dominated by the coexistence of the chalcocite-digenite solid solution with cobalt sulfides (CoS 2 , Co (sub 1-x) S, Co 4 S 3 ). As temperature is decreased to 880 degrees + or - 5 degrees C, the dominant ternary phase, the carrollite-linnaeite solid solution, appears with a composition of approximately Cu (sub 0.5) Co (sub 2.5) S 4 . The copper-rich compositional limit is extended on cooling, reaching a maximum (CuCo 2 S 4 ) at approximately 500 degrees C. Below 507 degrees C covellite is stable and coexists in the sulfur-rich portion of the system with copper-bearing cattierite. Below 500 degrees C the major changes involve a change in stable assemblages (at approximately 450 degrees C), in which the high-temperature pair Co + Cu 2 S is replaced by the pair Cu + Co 9 S 8 , the disappearance of Co (sub 1-x) S (< or =460 degrees C) and digenite (< or =70 degrees C), and the appearance of low chalcocite (103 degrees C), djurleite (93 degrees C), and anilite (70 degrees C). Optical and electron microprobe studies of natural assemblages permit speculation on phase relations at 25 degrees C and suggest the topology of the system is similar to that at 400 degrees C except for the appearance of the low-temperature copper sulfides. Similar studies also permit speculation on the low-temperature phase relations in the relevant portion of the Cu-Co-Fe-S system in which carrollite frequently occurs in close association with chalcopyrite or bornite and, in some cases, cobalt-pyrite. More cobalt-rich "carrollites" are found to occur in assemblages with more cobalt-rich "pyrites" and this corresponds to decreasing a (sub S 2 ) . Assemblages such as carrollite-chalco-pyrite-bornite-cobaltian pyrite and carrollite-cobaltian pyrite-bornite-digenite correspond to a further decrease in a (sub S 2 ) . The actual a (sub S 2 ) values during formation of assemblages containing these phases are also considered using thermochemical data for sulfidation equilibria. In the carrollite-linnaeite series, the systematic decrease in unit cell parameter and increase in reflectivity (at 589 nm) as a function of composition, as well as the actual compositional variations in the natural phases, are consistent with crystal chemical models.