Abstract <p>Samples of a monosulfide solid solution (<i>Mss</i>) based on α-NiS and FeS have been obtained at <i>P</i>&#xa0;=&#xa0;7.0 GPa and <i>T</i> = 900–1500°C with presynthesized sulfide compounds using the solid-phase method at <i>T</i> = 400–600°C. The structural and textural characteristics of the samples have been identified. Based on the features discovered and the chemical compositions of sulfide phases, a fragment of the phase diagram in the Fe–Ni–S system has been constructed with the assumed solidus and liquidus lines set. Thus, the obtained results show that in the studied range of compositions a complete series of solid solutions of minerals is formed. The evolution in the composition of the monosulfide solid solution and of the corresponding sulfide melt under diamond formation conditions as a function of temperature change has been traced. We have obtained preliminary estimates for the maximum Ni content in sulfide melts (up to 58 wt %), which may crystallize the minerals found in inclusions in natural diamonds of the peridotite paragenesis.</p>