The system Ni-S has been studied systematically from 200↶° to 1, 030↶° C by means of evacuated, sealed silica-glass tube experiments and differential thermal analyses. Compounds in the system are Ni3S2 (and a high temperature, non-quenchable Ni3↶±↶αS2 phase), Ni7S6, Ni1↶–S4 Ni3S4, and NiS2. The geologic occurrence of the minerals heazlewoodite (Ni2S2), millerite (↶βSNi1-2S), polydymite (Ni3S4), and vaesite (NiS2) can now be described in terms of the stability ranges of their synthetic equivalents. Hexagonal heazlewoodite, which is stoichiometric within the limit of error of the experiments, inverts on heating to a tetragonal or pseudotetragonal phase at 556↶° C. This high-temperature phase (Ni3 has a wide field of stability, from 23.5 to 30.5 wt per cent sulfur at 600↶° C, and melts incongruently at 806↶°↶±3↶° C. The ↶βNi7S6 phase inverts to ↶αNi78 at 397↶° C6 when in equilibrium with Ni3S2, and at 400↶° C when in equilibrium with ↶αNiS. Crystals of ↶αNi7S6 break down to Ni3-↶αS2↶+↶αNiS at 573↶°↶±3↶°C. The low-temperature form of Ni1-S1 corresponding to the mineral millerite, is rhombohedral, and the high-temperature form has the hexagonal NiAs structure. Stoichiometric NiS inverts at 379↶°↶±3↶°C, whereas Ni1-↶αS with the maximum nickel deficiency inverts at 282↶°↶±5OC. The Ni1-↶alphS-NiS2 solvus was determined to 985↶°↶±3↶°C, the eutectic temperature of these phases. Stoichiometric NiS is stable at 600↶°C but breaks down to Ni2-↶αS2 and ↶αNi1-S below 797↶°C, whereas ↶αNi1-S with 38.2 wt per cent sulfur melts congruently at 992↶°↶±3↶°C. Vaesite does not vary measurably from stoichiometric NiS2 composition, and melts congruently at 1.007↶±5↶°C. Polydymite breaks down to aNi-↶αS↶± vaesite at 356↶°↶±3↶°C. Differential thermal analyses showed the existence of a two-liquid field in the sulfur-rich portion of the system above 991↶°C and over a wide compositional range.