Tie line compositions of (Fe, Ni, Cu) (sub 1-x) S (monosulfide solid solution, mss) and Fe-Ni-Cu-S liquid, in the presence of sulfur vapor, have been quenched from temperatures between 1,050 degrees and 1,180 degrees C. More than 80 bulk compositions on both sulfur-rich and sulfur-poor sides of the mss field were investigated by sealed silica tube techniques.Qualitative observations of wetting behavior suggest increasing mobility in a silicate host rock, as sulfide liquids become more Cu rich. Partition coefficients for copper > or = 0.25, D (super mss/hq) Cu , are obtained from Ni-bearing experiments with approximately 2 wt percent Cu and no quench phases. The equation D (super mss/hq) Cu = 0.0003 X T( degrees C) + 0.0310 X (wt % S) + 0.0069 X (wt % Cu)--1.3450 describes partitioning observed in Ni-free experiments above 1,000 degrees C. Above 1,000 degrees C, the Ni distribution coefficient D (super mss/hq) Cu decreases with increasing temperature and/or sulfur content of the liquid.These results yield improved models describing the fractional crystallization of natural sulfide liquids. Major element (Fe, Ni, Cu, S) compositions of ores from the Sudbury district are shown to be entirely consistent with fractional crystallization at temperatures above 1,000 degrees C, with the possible exception of rare samples enriched in both Ni and Cu. Sulfide liquids fractionating Ni-Fe-rich hanging-wall ores at Sudbury, Ontario, must have been less Cu rich than previously thought. At temperatures above 950 degrees C, reduced sulfur activity in residual liquids can result in massive late-stage bornite-rich ores.