An elongate field of high-form pentlandite solid-solution, Fe5.65Ni3.35S7.85, β2 (Ni4±xS3), occurs in the system Fe–Ni–S at 650°C. This solid solution coexists with monosulfide solid-solution, β1 (Ni,Fe)3±xS2 and γ (Fe,Ni). Pentlandite with a composition Fe5.60Ni3.40S7.82 first appears as a stable phase at 625°C owing to the phase transition of the most Fe-rich high-form pentlandite with the same composition. It grows as a limited solid-solution, from Fe5.64Ni3.36S7.82 to Fe3.25Ni5.75S7.92 at 600°C and from Fe5.68Ni3.32S7.85 to Fe2.43Ni6.57S7.85 at 500°C owing to a continuous phase-transition, exsolution and breakdown (pseudoperitectoid and pseudo-eutectoid) of the high-form solid-solution and the exsolution and breakdown (pseudo-eutectoid) of β1. The compositional range of the solid solution is also increased by the exsolution of monosulfide solid-solution below 625°C. Pentlandite coexists with high-form pentlandite (625° to 503°C), monosulfide solid-solution (below 625°C), γ (below 617°C) and β1 (579° to 484°C). High-form pentlandite still remains stable below 520°C, but breaks down to pentlandite, high-form godlevskite and β1 at 503° ± 3°C and Fe1.04Ni7.96S6.93 (eutectoid). Phase β1 also breaks down to pentlandite, heazlewoodite and γ at 484° ± 3°C and Fe0.26Ni2.87S2.00 (eutectoid). The assemblages with pentlandite and high-form godlevskite or heazlewoodite first appear at 568° ± 3°C or 498° ± 3°C, respectively. In this study, we show that pentlandite in the Ni–Cu ores can form at temperatures from 625° to 500°C or less owing to the phase transition, exsolution and eutectoid of the high-form pentlandite solid-solution, monosulfide solid-solution and β1. These are the primary phases that would crystallize from sulfide magma (liquid in the system Fe–Ni–S) between around 1000° and 750°C.