In conjunction with a study of trace elements, about 200 sphalerites from the Central district, New Mexico, and the Bingham district, Utah, have been analyzed for Fe by an X-ray fluorescence method. The FeS content ranges from 4.2 to 18.5% at Hanover in the Central district and from 0.84 to 9.4% in the southwestern part of the same district. At Bingham the range is from 0.2 to 16.7%. Pyrite is ubiquitous in both districts, and pyrrhotite is known. Sphalerite with pyrrhotite averages higher in Fe content than sphalerite with pyrite. A poorly-developed lateral zoning away from igneous centers is present in the Central district and poorly-developed vertical zoning was found in one structure at Bingham. Ten analyses of a zoned crystal from the Central district show a range from 4.3 to 12.1% FeS, and sharp boundaries exist between the zones, indicating that equilibrium was not achieved at temperatures of at least 425 degrees C. Several other inhomogeneous sphalerites were found in both districts, and lack of equilibrium is believed to be a major difficulty in the determination of temperature from the Fe content of sphalerite. Other difficulties include the effect of S pressure, the total pressure correction, and exsolution effects. Because all these factors probably result in a lower Fe content than is indicated by the FeS-ZnS solvus for one atmosphere, it is suggested that temperatures derived from the Fe content of sphalerite be regarded as minimum temperatures unless convincing evidence can be presented on the possible complications. On this basis, temperatures reached at least 600 degrees C. at Hanover in the Central district, at least 350 degrees C. at the Groundhog mine in the same district, at least 650 degrees C. at the Cleveland mine in the nearby Pinos Altos district, and at least 550 degrees C. in the Bingham district. At the Cleveland mine, higher than normal CO 2 pressures seem necessary to explain the lack of reaction between quartz and carbonates, if the sphalerite temperatures are correct.