The equilibrium diagram for the Fe-Zn-S System has been worked out in détail from 580° to 850° C. Previous work on this System is proven to be seriously in error and températures heretofore estimated from the "sphalerite geothermometer" are without a sound quantitative foundation. Sphalerite solid solutions lie essentially along the FeS-ZnS join. Neither pyrite nor pyrrhotite takes up appréciable amounts of zinc. The FeS-ZnS binary solvus (équivalent to the assemblage sphalerite + stoichi-ometric FeS) is very steep, passing through 56 mole percent FeS at 850° C and 52 mole percent FeS at 580° C. It is not useful geologically because of the rarity of stoichiometric FeS as a primary minerai in sphalerite-bearing assemblages; however, the relatively large rôle played by total pressure on the binary solvus might permit the iron content of sphalerite to be useful in determining the pressure of équilibration of some météorites. Of greater géologie interest is the sphalerite + pyrite + pyrrhotite assemblage in which the composition of sphalerite changes from 13 mole percent FeS at 742° C to 19 percent at 580° C. Unfortunately, this equilibrium cannot be quantitatively extrapolated to lower températures from the presently available data and therefore quantitative geothermom-etry of pyrite + pyrrhotite + sphalerite assemblages is not feasible at présent. Thermochemical calculations prove that the iron content of sphalerite in pyrite + sphalerite assemblages is very sensitive to variations in température and fugacity of S2; this relationship provides an explanation of growth-zoned sphalerite as the resuit of fluctuations of sul-fur fugacity, and, in combination with other sulfide assemblages that define isobarically univariant relations between température and sulfur fugacity, forms a potential basis for quantitative geothermometry.