The order-disorder transformation behavior of chalcocite and djurleite, and digenite and anilite has been studied by direct observations of the transformations in a transmission electron microscope. Chalcocite and djurleite represent low-temperature superstructures of the high-temperature disordered chalcocite subcell, and there need be no compositional difference between them. The djurleite superstructure is the more stable form at room temperature. The intermediate structure in digenite forms through modulated structures rather than through twinning. A metastable 6a digenite type superstructure, isochemical with anilite, forms on heating. This 6a superstructure is metastable relative to the formation of anilite.