A reconnaissance study of the phase relations involving sphalerite, pyrite, pyrrhotite, intermediate solid solution, and coexisting phases in the central portion of the Cu-Fe-Zn-S system was conducted for the interval 800 degrees to 500 degrees C. This report discusses sphalerite relationships in this interval. The copper content of sphalerite, which is dependent upon temperature and the activity of FeS, may exceed 5 wt percent when coexisting with pyrrhotite and intermediate solid solution and 7 wt percent when in equilibrium with bornite. The exsolution of the copper and iron as a sulfide or sulfides does not appear to alter significantly the utility of the sphalerite geobarometer. The solid solution of CuS in iron-bearing sphalerite slightly reduces the unit cell dimension. Natural sphalerites, though, after initial equilibration with copper and iron sulfides, have reequilibrated by exsolution and now invariably contain less than 0.5 wt percent CuS.