The thermal stability of pentlandite was studied by quenching, differential thermal analysis, and high-temperature X-ray diffraction experiments. Rigid silica tubes were employed as reaction vessels, and vapor, therefore, is an inherent phase in all 3 types of experiments. Synthetic pentlandite of Fe (sub 4.5) Ni (sub 4.5) S 8 composition to decompose at 610 degrees + or - 2 degrees C. to (Fe, Ni) (sub 1-x) S with hexagonal pyrrhotite structure and to a high-temperature nonquenchable phase, equivalent to Ni (sub 3+ or -x) S 2 of the pure Ni-S system, but containing some Fe. Of these phases (Fe, Ni) (sub 1-x) S has low Ni content and a metal-to-S ratio of 9:10 or possibly slightly higher and the Ni (sub 3+ or -x) S 2 phase has a metal-to-S ratio of 3:2 or slightly less. The first appearance of liquid was recorded at 862 degrees C. in differential thermal analysis experiment on synthetic material of Fe4.5) Ni (sub 4.5) S 8 composition. This temperature corresponds to that given in the literature as the "melting point" for pentlandite. Experiments on pentlandite from Frood Mine, Sudbury [Ontario], show breakdown of pentlandite at 613 degrees C. and melting effects in the temperature region 864 degrees to 881 degrees C. These experimental results have important bearing on the geological interpretation of pentlandite-pyrrhotite assemblages in ores such as those of the Subury area. In the pure Fe-Ni-S system a field containing homogeneous liquid of composition pertinent to the formation of pentlandite-pyrrhotite or pentlandite-heazlewoodite assemblages exists above approximately 1000-1100 degrees C. On cooling Fe, Ni) (sub 1-x) S first crystallizes from the liquid. This phase coexists down to 862 degrees C. with a metal-rich Fe-Ni-S liquid. Below 862 degrees C., but above 610 degrees C., the (Fe, Ni) (sub 1-x) S phase coexists with (Ni, Fe) (sub 3+ or -X) S 2 . At 610 degrees C. pentlandite appears and the (Fe, Ni) (sub 1-x) S-(Ni,Fe)3+ or -x) S 2 assemblage is no longer stable. At lower temperatures, depending upon bulk composition, the pyrrhotite-pentlandite or pentlandite-(Ni, Fe) (sub 3+ or -x) S 2 pairs are stable. The (Ni, Fe) (sub 3+ or -x) S 2 phase breaks down on cooling below about 550 degrees C., and depending upon the metal-to-S ratio, heazlewoodite and pentlandite, or heazlewoodite, awaruite, and pentlandite, or heazlewoodite, millerite (or Ni 7 S 6 ), and pentlandite will form stable assemblages.