A detailed study of the central portion of the Fe-Ni-S system between 600 degrees and 250 degrees C has been conducted by means of silica-tube techniques. Particular emphasis has been placed on defining the limits of the mono-sulfide solid solution (Mss) that extends across the system from Fe (sub 1-x) S to Ni (sub 1-x) S; the sulfur-poor limit is significant with respect to the formation of pentlandite in iron-nickel sulfide ores.At 600 degrees C the Mss appears on the phase diagram as a nearly straight-sided band, thinner at the nickel-rich than at the iron-rich end. The band becomes narrower as the temperature falls: the sulfur-rich limit remains nearly straight but moves towards the sulfur-poor limit; the sulfur-poor limit moves rapidly towards the sulfur-rich limit in the center but not at the ends of the solid solution, so that at lower temperatures the Mss is concave outwards on the sulfur-poor side. The temperature at which the Mss becomes incomplete and below which the pyrite-pentlandite assemblage is stabilized is not known precisely, although it is below 300 degrees C.Magnetite and chalcopyrite, which often occur with pyrrhotite and pentlandite in ores, do not change significantly the temperature at which pentlandite exsolves from Mss of any given composition. When projected from chalcopyrite onto the Fe-Ni-S face of the Cu-Fe-Ni-S system, compositions of most pyrrhotite-pentlandite-chalcopyrite ores fall within the boundaries of the Mss at 500 degrees C. If these deposits were formed as equilibrium assemblages at or above this temperature, the pentlandite must have exsolved from Mss.The shape of the pentlandite-pyrrhotite solvus is such that pentlandite exsolution is more dependent on the metal: sulfur ratio than the nickel content of the Mss. Two copper-poor deposits serve as examples: pentlandite exsolution would have started at the Alexo deposit ( approximately 6.5 percent Ni in sulfides, 38-38.2 percent S in pyrrhotite) at 350 degrees -400 degrees C, but in the massive ore at the Marbridge deposit ( approximately 6.5 percent Ni in sulfides, 39.2-39.8 percent S in pyrrhotite) it would not have begun above 300 degrees C. Since most if not all of the nickel in iron-nickel sulfide deposits is in solid solution as nickeliferous pyrrhotite at temperatures as low as 300 degrees C, the pyrrhotite-pyrite solvus is invalidated as a geothermometer in these ores.