Abstract The phase relations in the Cu-Fe-Ni-S system are discussed in the light of experimental results from liquidus temperatures to below 550° C. Although liquidus temperatures vary greatly throughout the system, beginning of melting of a typical ore composition (1.5% Cu, 4.5% Ni, 38.5% S, 55.5% Fe) was found to occur at 1,105° C. Magnetite lowers the beginning-of-melting temperature to 1,019° C. In contrast, water influences the beginning-of-melting temperature by less than 20° C. Under conditions encountered in typical massive Ni-Cu sulfides fractionation leading to formation of a Cu-enriched liquid is possible only above 850° C. Ni enrichment in a liquid phase is not encountered below 1,000° C except under sulfur pressure conditions much lower than those encountered in typical Ni-Cu ores. The bulk compositions of Ni-Cu sulfide ores commonly lie within the confines of a quaternary monosulfide solid solution at temperatures above 500° C. This suggests that such ores formed a homogenous phase at the time of their emplacement. Cooling below 600° C may result in exsolution of chalcopyrite and/or pyrite or chalcopyrite and/or pentlandite depending on sulfur pressure. The commonly observed pentlandite +pyrite assemblage is stable only well below 300° C and cannot crystallize directly from a Cu-Fe-Ni sulfide melt. The absence of natural analogues even of a ternary (Fe,Ni) 1–x S monosulfide solid solution and the frequent occurrence of monoclinic pyrrhotite and the pyrite + pentlandite assemblage indicate low-temperature reequilibration of mineral assemblages and mineral compositions subsequent to original emplacement which may have occurred at or near liquidus temperatures. Such reequilibration negates usage of the pyrrhotite-pyrite geothermometer in magmatic Ni-Cu ores. The present study offers a new view of the paragenesis of Ni-Cu ores and demonstrates that the observed mineral assemblages may be explained in terms of phase changes occurring in a system of isochemical bulk composition in response to decreasing temperature.