New X-ray and microscope data on phases in the range Cu (sub 1.73) to Cu 2 S, at temperatures up to 700 degrees C, cover the critical part of the system Cu-S. Phases stable at low temperature include digenite (to 76 degrees C for Cu (sub 1.765) S, 83 degrees C for Cu (sub 1.840) S), djurleite (to about 93 degrees C), and orthorhombic chalcocite (to 93 degrees C for Cu (sub 1.993) S, 103.5 degrees for Cu 2 S). Within the range Cu (sub 1.73) S to Cu 2 S, high-digenite solid solution is the only stable phase at 507 degrees C (upper stability limit of covellite). Hexagonal chalcocite inverts to high digenite (Cu 2 S) at about 435 degrees C. Natural stable assemblages agree with experimental data. The lower stability limit of high digenite has qualified usefulness as a geothermometer.