Phase equilibria in the system Cu-GaS were investigated by dry synthesis at 500° and 400°C using the three elements (Cu, Ga and S) and four synthetic sulfi des (CuS, Cu 2 S, GaS and Ga 2 S 3) as starting materials; we established the phase diagrams for this ternary system at both temperatures. Gallite, the only reported mineral in the system, is located in the central position in these diagrams, and many tie lines extend from it to other phases; covellite, digenite, phases ␣, , ␥, GaS, X and and sulfur. Gallite shows essentially no solid solution with the other phases, appearing as stoichiometric CuGaS 2. On the join between gallite and Ga 2 S 3 , a new phase, named X, was found. It is whitish gray with yellow internal refl ections in refl ected light. It is optically very similar to Ga 2 S 3 , but its X-ray powder-diffraction data show a tetragonal symmetry. The cell parameters are a 2.250, c 10.474 Å. It has a limited solid-solution fi eld ranging from Cu 10.0 Ga 31.7 S 58.3 to Cu 10.5 Ga 32.5 S 57.0 at 500°C and from Cu 9.7 Ga 32.2 S 58.1 to Cu 10.4 Ga 32.2 S 57.3 at 400°C. Digenite contains between 0.2 and 0.4 at.% Ga, GaS contains from 1.1 to 1.4 at.% Cu, and Ga 2 S 3 contains from 0.9 to 1.0 at.% Cu at both temperatures. Along the Cu-Ga join, fi ve phases, labeled ␣, , ␥, and gallium liquid, exist at both temperatures. The phases ␣, , and ␥ are connected to gallite with tie lines, and the phases ␥, , and gallium liquid are connected to GaS with tie lines at both temperatures.