A systematic x-ray study of the solid phases of the system Cu 2 S-CuS has been made. A special camera was designed to make x-ray photographs of powder samples at elevated temperatures. This camera allows the specimen to remain heated continuously, and the heat treatment can be controlled as in a furnace. The resulting phase diagram shows that the system contains three compounds and four phases. The compounds are:chalcocite, ideally Cu 2 Sdigenite, ideally Cu 9 S 5 covellite, CuS.Chalcocite undergoes transformation, and no cubic high temperature phase appears up to at least 300 degrees C. The revised inversion scheme is:above 105 degrees C., hexagonal basic structurebelow 105 degrees C., orthorhombic superstructure.The superstructure phase is capable of dissolving up to eight atomic per cent CuS, while the basic structure can dissolve only two atomic per cent CuS. Below approximately 78 degrees C. digenite has the ideal composition Cu 9 S 5 , but above this temperature it takes increasing amounts of either Cu 2 S or CuS into its composition. Evidently digenite has been regarded as the phase of chalcocite stable above 91 degrees C. Certain regions of the phase diagram of this system may be applied to problems of geologic thermometry, providing that proper criteria are recognized.