Compositions on the join Cu 2 S-Ag 2 S were studied with X-ray diffractometer heating stage. Low-temperature phases, which break down at temperatures between 67 degrees and 119 degrees C, are chalcocite, stromeyerite, Cu (sub O.8) Ag (sub 1.2) S, jalpaite, and acanthite. High-temperature phases, all cation-disordered, are hexagonal close-packed (high chalcocite), face-centered cubic (high digenite), and body-centered cubic (argentite) in order of increasing Ag. Above 593 degrees C, the face-centered cubic phase occupies the whole field of the join, extending also to more sulfur-rich compositions. Although reaction rates are extremely rapid in the system, and the high-temperature phases are unquenchable, pseudomorphs and textures in natural specimens can be informative.