Sphalerite, pyrite, and pyrrhotite were recrystallized together in a saturated ammonium iodide solution at 150 degrees to 325 degrees C and 0.4 to 5.1 kbars. The data were obtained from ten sets of experiments comprising gold capsules stacked consecutively within the bores of cold seal pressure vessels which were heated for periods of between 300 and 750 days. Five capsules contained magnetite as an additional reactant.A P-T-composition map has been constructed for sphalerite buffered by pyrite and hexagonal pyrrhotite. The upper P-T range for the present study includes the lower temperature boundary for the sphalerite geobarometer which has been located near 285 degrees C at 0.5 kbars and 300 degrees C at 5 kbars. The provisional lower P-T range for the present data extends from approximately 180 degrees C at 0.5 kbars to 190 degrees C at 5 kbars. Within the investigated P-T interval, isobaric composition curves for sphalerite show a progressive decrease in iron content with increasing pressure and decreasing temperature.Hexagonal pyrrhotite was the only iron monosulfide identified in the experimental reaction products and was found to increase in iron content with increasing pressure. Its occurrence in these long-term experiments supports earlier predictions that monoclinic pyrrhotite may be a metastable phase down to low temperatures. Hexagonal pyrrhotite may stably invert to monoclinic pyrrhotite at temperatures below 150 degrees C under isotropic stress conditions; however, at higher temperatures monoclinic pyrrhotite appears to be favored by anisotropic stress conditions which produce lattice strain. Under these conditions it may form and persist metastably as the preferred phase below the sphalerite geobarometer boundary. The presence of magnetite does not affect the compositions of sphalerite or pyrrhotite under the conditions of study.The present calibration may be applied to hydrothermal vein deposits provided codeposition of phases can be proven; however, it is expected to have limited application for massive sulfide ores that have prograded or retrograded to temperatures around or below 300 degrees C. This is because the calibration requires buffering by pyrite and hexagonal pyrrhotite, whereas most lower temperature assemblages appear to have been buffered by pyrite and monoclinic pyrrhotite over a range of temperatures, pressures, and anisotropic stress conditions. This latter buffer has restricted sphalerite compositions between around 11 and 12 mole percent FeS.