The phase relations in the central portion of the Cu-Fe-S system were studied under hydrothermal conditions by the thermal gradient transport method. Isothermal sections at 350 degrees C and 300 degrees C are presented, and the extent of the bornite, chalcopyrite, pyrrhotite, and intermediate solid solution fields, the composition of idaite, and their phase relations are discussed. The extensive intermediate solid solution in the center of the system at high temperatures separates into tetragonal chalcopyrite and an isometric intermediate solid solution. At 350 degrees C, chalcopyrite has a narrow solid solution field extending from the stoichiometric CuFeS 2 to Cu (sub 0.9) Fe (sub 1.1) S (sub 2.0) (Cu/Fe atomic ratio of 1.00 to 0.87) along a line with metal/sulfur atomic ratio of approximately 1, while the isometric solid solution extends from nearly stoichiometric CuFe 2 S 3 to Cu (sub 1.2) Fe (sub 1.1) S (sub 2.0) (Cu/Fe atomic ratio of 0.49 to 1.08). The bornite solid solution extends farover stoichiometric Cu 5 FeS 4 toward an iron-rich composition, and the Cu/Fe atomic ratio reaches 3.2 at 300 degrees C and 2.9 at 350 degrees C. The compositional range of bornite in equilibrium with chalcopyrite is very wide, varying from approximately 4.0 to 7.5 in Cu/Fe atomic ratio. The pyrrhotite solid solution has some extent in the ternary field at high temperatures but retreats rapidly to the binary Fe-S join as the temperature decreases and the maximum content of copper in pyrrhotite is only 0.6 weight percent at 350 degrees C and 0.3 weight percent at 300 degrees C. Idaite has no solid solution field, and the composition is approximately Cu = 57.0, Fe = 9.1, and S = 33.9 in weight percent, very close to Cu (sub 5.5) FeS (sub 6.5) as proposed by Yund (1963). It was established that the idaite-chalcopyrite tie line, being apparently incompatible with the previously accepted bornite-pyrite assemblage, exists stably at 350 degrees C and 300 degrees C under hydrothermal conditions. It was also confirmed that the tie line change from the intermediate solid solution-pyrite into chalcopyrite-pyrrhotite occurs at 328 + or - 5 degrees C as had been suggested by Yund and Kullerud (1966).