We have been exploring ways to quantitatively assess the extent to which fractionation of sulfide melt has effected variations in composition within magmatic sulfide ore bodies. Our approach has been to determine by experiment the crystallization paths of sulfide liquids in temperature and composition dimensions. In this paper, the results of new major-element partitioning experiments below 1050 °C in the nickel-free system are presented and summarized along with new and previous work in the Fe–Ni–Cu–S quaternary. The partition coefficients D for Cu between monosulfid solid solution (mss) and sulfide liquid in the Ni-free system (DCu = (wt.% Cu in mss)/(wt.% Cu in liquid)) cluster near 0.3, but decrease to nearly 0.1 for Cu-rich, S-poor liquids near 1000 °C. DNi also declines with decreasing sulfur content of the liquid, but increases with decreasing temperature. Preliminary data indicate that DNi exceeds 1.0 in low-Ni liquids with greater than 16 wt.% Cu, at 1050 °C. The quality of available data on the Fe–Ni–Cu–S system currently exceeds the sensitivity of crystallization models based on the distribution coefficient approximation for major elements. However, we present equations for variable distribution coefficients for Ni and Cu that can be incorporated into calculations of the ratio of trapped initial liquid to fractionated solid for bulk ore samples, using D values for platinum group elements from the literature. Fractionation can then be modeled quite well using an iterative approach, with D values changing in response to liquid composition with each increment of crystallization along an assumed temperature path.