Phase relations in the Cu-Fe-Zn-S system were studied between 500 degrees and 300 degrees C using both the thermal gradient transport method and the isothermal in situ recrystallization method under hydrothermal conditions. In addition, the compositional fields of chalcopyrite, intermediate solid solution (iss), and sphalerite were determined at temperatures between 500 degrees and 300 degrees C.The stable univariant phase assemblages in the central portion of the system at both 500 degrees and 400 degrees C are covellite + nukundamite + pyrite + sphalerite, covellite + bornite + nukundamite + sphalerite, bornite + nukundamite + pyrite + sphalerite, bornite + chalcopyrite + pyrite + sphalerite, bornite + chalcopyrite + iss + sphalerite, chalcopyrite + iss + pyrite + sphalerite, and iss + pyrite + pyrrhotite + sphalerite. At 300 degrees C stable assemblages of chalcopyrite + pyrite + pyrrhotite + sphalerite and chalcopyrite + iss + pyrrhotite + sphalerite appear in place of the last two assemblages at 500 degrees and 400 degrees C because of a tie line between chalcopyrite and pyrrhotite. Also, a stable tie line between bornite and pyrite commonly found in natural ores occurs in the present system instead of the tie line between nukundamite and chalcopyrite observed in the Cu-Fe-S system.Chalcopyrite has a very small solid solution field close to the stoichiometric CuFeS 2 and dissolves small amounts of zinc (<0.9 at. %) at all three temperatures. Intermediate solid solution has an extensive solid solution field which dissolves up to 3.3 at. percent at 500 degrees C and 1.2 at. percent at 300 degrees C, with a slight reduction in solubility due to pressure. The solid solution area of intermediate solid solution becomes narrower as the temperature decreases and becomes richer in copper with increasing pressure as shown in the system Cu-Fe-S by Sugaki et al. (1982). Sphalerite solid solution dissolves a maximum of about 2.4 mole percent CuS at the three temperatures and has no relation to FeS content and sulfur fugacity.