A basis for the interpretation of Fe-As-S mineral assemblages in ores has been provided through systematic collection of physical and chemical data in the synthetic system Fe-As-S. Most interesting is the temperature limitation of 491 degrees C. for coexistence of the commonly observed mineral pair pyrite-arsenopyrite. The As:S ratio in arsenopyrite and/or loellingite may add another to the group of geothermometry techniques now available for ore studies. The observation that Au diffuses rapidly through fine-grained arsenopyrite at temperatures above 600 degrees C. and confining pressures up to 2,070 bars provides some insight into the relations of Au and arsenopyrite in many ores. Equilibrium phase relations in the Fe-As-S system were determined at 600 degrees C., and changes in assemblages were studied in the 400 degrees to 800 degrees C. temperature range. At 600 degrees C. a very narrow liquid field lies along the As-S side of the ternary system between 100 and 22.8 + or - 0.2 weight percent S. Tie-lines connect various parts of this liquid field to pyrite, to pyrrhotite, and to arsenopyrite. At this temperature there are also tie-lines between pyrrhotite-arsenopyrite, arsenopyrite-As, arsenopyrite-toellingite, pyrrhotite-loellingite, pyrrhotite-FeAs, and FeS-Fe 2 As. At temperatures above 600 degrees C. synthetic arsenopyrite has the approximate composition FeAs (sub 1.1) S (sub 0.9) . Compositions that are S rich relative to ideal FeAsS become stable at lower temperatures and under high confining pressures. Changes in the phase assemblages at various temperatures are governed by the reactions: pyrite + arsenopyrite <--> pyrrhotite + liquid or vapor, arsenopyrite + As <--> loellingite + liquid or vapor, and arsenopyrite <--> pyrrhotite + loellingite + liquid or vapor. The invariant temperature at which both liquid and vapor are present in these assemblages are 491 degrees + or - 12 degrees C., 688 degrees + or - 3 degrees C., and 702 degrees + or - 3DGC., respectively. The first reaction was investigated at confining pressures up to 2,070 bars, at which pressure pyrite and arsenopyrite can coexist up to 528 degrees + or - 10 degrees C.