The S-rich portion of the Fe-Ni-S system has been investigated between 100 degrees and 1000 degrees C. under equilibrium vapor pressures with especial emphasis on the FeS 2 -NiS 2 join. Pyrite (FeS 2 ) and vaesite (NiS 2 ) coexist between 729 degrees + or - 3 degrees and 137 degrees + or - 6 degrees C. Above this temperature range, a (Fe, Ni) (sub 1-x) S solid solution coexists with S liquid, whereas below 137 degrees C. bravoite [(Fe, Ni)S 2 ] is stable on the join between pyrite and vaesite. The solubility of FeS 2 in vaesite is a maximum of 27.9 wt % at 729 degrees C.; other points on the solvus are 13.5% at 600 degrees C, 6.5% at 500 degrees C, 2.5% at 400 degrees , 0.7% at 300 degrees , and 0.1% at 200 degrees C. The heat of mixing, delta H, is constant at 9.8 kcal/mole. The maximum NiS 2 solubility in pyrite of 7.7 wt % also occurs at 729 degrees C. It decreases to about 6.8% at 700 degrees C. but could not be determined at lower temperatures. When pyrite and vaesite coexist, the vaesite solvus is a useful geothermometer. A determination on material from the Kasompi Mine, Katanga, indicates a formation temperature of 390 degrees C. Bravoite, generally a secondary mineral, can form only below 137 degrees C. C. Most nickeliferous pyrites were deposited at relatively low temperatures where the equilibrium Ni solubility is well below 1%. It is concluded, in the light of the experimental evidence, that most of the Ni occurs in metastable solid solution in these pyrites.