The phase relations in the Fe-Sb-S system from 300 degrees to 800 degrees C have been determined using new experimental data on phase assemblages and univariant equilibria in conjunction with information from the literature. Estimates have been made for the standard free energies of formation as a function of temperature for stibnite, berthierite, gudmundite, and FeSb 2 . Comparison of calculated mineral stabilities with published information on mineral assemblages suggests that apparently contradictory mineral associations may well represent equilibration at different (but all low) temperatures with subsequent lack of reaction as temperature fell to the temperature of observation. Resolution of the complicated, low-temperature phase relations will require additional thermochemical data of unusually high accuracy that might well be supplemented by careful study of natural materials.Berthierite, gudmundite, and aurostibite are clearly stable only under conditions of relatively low sulfur activity. FeSb 2 requires such low sulfur activity for its stable existence that it is seldom, if ever, to be expected as a mineral. The tendency for the common ore mineral galena to react with berthierite to yield pyrrhotite plus a lead sulfantimonide operates to make berthierite a rather uncommon mineral.