Cu-Ni ores from seven mines in the Sudbury ore camp have been analyzed. Together with older analyses, they indicate a weighted mean δ34S for all ore of + 2.1‰ (CDT), a range from + 0.5 to 7.2‰. Individual deposits have characteristic δ34S means, and small dispersions compared to whole camp. 34S-content is not correlated with ore type, as defined by sulfide-silicate association; δ34S increases with depth in south-range deposits; north-range ores are heavier still, as if deposits had been uplifted relative to south-range, contrary to conventional structural interpretation.Fractionation between pyrrhotite and chalcopyrite (Δpc = δSPo – δScp) ranges from − 0.8 to + 0.9‰. Negative fractionations may be quenched in from high temperatures. Δpc is positively correlated with δSpo. Both variables may be responding to increasing depth of emplacement, δSpo by progressive fractionation through extraction of S-32 enriched crystals from sulfide magma; Δpc through decreased cooling (quenching) rate. Alternately, Δpc – δpo variations may represent partial exchange of pyrrhotite with an 32S-rich fluid after ore deposition.Overall δS of ore suggests that primary sulfide magma is fractionated with respect to primitive basaltic sulfur, possibly by loss of S-32 enriched vapor.