The composition of any sample of a magmatic sulfide ore depends on the composition of the silicate magma from which the sulfide segregated, the appropriate sulfide liquid-silicate magma partition coefficients, the "R" or "N" factor operating during the interaction of the sulfide and silicate host, the subsequent fractionation of the resulting sulfide liquid, and the proportion of cumulus minerals to sulfide liquid represented within the sample. All these factors have been considered during the investigation of platinum-group elements (PGE) in 2,500 samples representative of 12 mineralized systems (ore deposits or areas of mineralization) at Sudbury. The data indicate that samples from most of these systems fit within envelopes defined by the compositions of cumulus mss and the associated sulfide liquid. Once the fractionating liquid reaches about 32 wt percent Cu, iss replaces mss as the liquidus phase, and Rh and Rh/Cu increase with continued fractionation. The initial composition of the sulfide liquids responsible for each of the mineralized zones has been determined by adjusting the position of the mss-sulfide liquid envelope to best include the data for the zone on an Rh/Cu versus Rh plot. The initial liquid compositions determined in this way vary widely, and, for most deposits, can be explained as the result of the equilibration of sulfide liquid with the same magma at R factors ranging from 1,200 (i.e., the liquids with the highest Ni, Cu, and PGE) to 140 (i.e., the liquids with the lowest Ni, Cu, and PGE). Deposits along the South Range are characterized by sulfide liquids with a distinctly higher R factor than those along the North Range. The data suggest that two deposits, Creighton and Victor, cannot be explained as the result of fractionation of a single sulfide liquid but that two or more liquids with different R factors were involved. The data also show that the mss-sulfide liquid partition coefficients for Rh, Ir, and Os are 4, 4.4 to 5.2, and 4.2, respectively, essentially confirming the experimentally determined coefficients reported in the recent literature. In the Trillabelle mineralized system, sulfide within sublayer norite started crystallizing mss before the sublayer norite itself was completely solidified; where the sulfide content exceeded 20 percent sulfide, Cu-rich, fractionated liquid was able to escape, leaving behind mss cumulates. In most areas with less than 15 percent sulfide, fractionated liquid has not escaped, and the sulfides have a composition close to that of the original sulfide liquid. Examination of a zone of subeconomic mineralization in this way can help assess the likelihood of there being high-grade zones of Cu- and PGE-rich fractionated sulfides in the vicinity. Comparison of the estimated composition of the sulfide liquid obtained from the sulfide-poor samples with that obtained from fitting a fractionation envelope to the sulfide-rich samples indicates that the rich mineralization crystallized from a liquid with a composition different from that found in much of the sublayer and mafic norites. At the Lindsley deposit, there is a close correlation between the presence in the footwall of a pod of massive sulfide with a fractionated bulk composition and sulfides within the adjacent sublayer norite that are "wetter" (i.e., have a higher phi index) than is typical for the greater part of the sublayer norite mineralization at the deposit, suggesting that fractionating sulfide liquid has moved through the sublayer norite, leaving behind "dry" cumulus mss and a "wet" trail at the location where it leaked out into the footwall.