Phase relations have been determined among sphalerite, pyrite, pyrrhotite, and intermediate solid solution (iss) from 439 degrees to 725 degrees C at 1 bar and 353 degrees to 760 degrees C at 5 kb. The extents of solid solution in the four-phase assemblage are described as functions of temperature (in Kelvins) by the following equations: log mole % ZnS in iss (1 bar) = 2.795 - 1,632 T (super -1) , log mole % ZnS in iss (5 kb) = 2,659 - 1,608 T (super -1) , log mole % CuS in pyrrhotite (1 bar) = 2,546 - 1,866 T (super -1) , log mole % CuS in sphalerite (1 bar) = 4.202 - 3,755 T (super -1) , and log mole % CuS in sphalerite (5 kb) = 4.084 - 3,791 T (super -1) . The pressure dependence (in kb) of the composition of sphalerite in equilibrium with pyrite and pyrrhotite with or without iss can be expressed for the temperature-independent portion of the sphalerite geobarometer as P = 42.30 - 32.10 log mole % FeS.Because of the low solubility of copper in sphalerite in the four-phase assemblage below 650 degrees C, the presence of chalcopyrite in natural ores should be of no consequence to sphalerite geobarometry over its temperature-independent portion. Nevertheless, sphalerites which are inhomogeneous in FeS and which contain abundant blebs of chalcopyrite in excess of the amount that can be reasonably accounted for by an equilibrium process of solubility and subsequent exsolution are a common feature of many sulfide ores. Such sphalerite grains should be avoided in applying the geobarometer. The chalcopyrite blebs may be a product of a premetamorphic sphalerite-chalcopyrite intergrowth or represent true exsolution from sphalerite in any one of (1) a former sphalerite + pyrrhotite + iss equilibrium, (2) a stable four-phase assemblage at temperatures above the temperature-independent portion of the geobarometer, or (3) a metastable four-phase assemblage in which pyrite was inert. Use of the sphalerite geobarometer requires a thorough understanding of the textural relationships and discrimination of equilibrium from disequilibrium or low-temperature reequilibrated assemblages.