Sulfur isotope measurements provide a useful means of evaluating the source of sulfur in magmatic Cu-Ni-PGE deposits. Although low sulfur, PGE-rich deposits (e.g., J-M reef of the Stillwater Complex, Merensky reef of the Bushveld Complex) show little or no isotopic evidence for the involvement of crustally derived sulfur, several high sulfur deposits (e.g., Noril’sk, Duluth Complex) show strong evidence for contamination of mafic magmas by sulfur of crustal origin. Processes such as magma degassing or redox changes accompanying crystallization are ineffective in significantly modifying sulfur isotope signatures of sulfides that crystallize from immiscible liquids that exsolve from the mafic magmas. For this reason δ34S values are primarily dependent on sulfur source characteristics, and variations can often be treated using a two-component mixing approach. However, deposits such as those at Noril’sk and Voisey’s Bay that appear to have formed in magma conduit systems present another possibility for modification of sulfur isotope signatures. These deposits are thought to have been upgraded in metal tenor by reaction with uncontaminated mantle-derived melts that traversed the accumulated sulfide on their way to the surface. Our calculations illustrate the efficiency of reactions that involve the exchange of Fe for Cu, Ni, or PGE in producing increases in the metal tenor of the accumulated sulfide. Sulfur isotope exchange between the crustally contaminated sulfide ore accumulation and sulfur of mantle origin may accompany chalcophile element exchange, and is capable of masking or limiting the degree of isotopic evidence for the initial stage of ore genesis that involved crustal sulfur. Sulfur isotope exchange is distinct from the addition of sulfur ( in the form of immiscible sulfide droplets from a sulfide-saturated magma) to the accumulated sulfide, although both additive mixing and exchange can lead to similar isotopic effects. Stable isotopic exchange provides a mechanism in addition to additive mixing that may lead to decoupling of different isotopic systems.