Measurements of the Fe and Ni content of olivine and coexisting bulk sulfide from the different sulfide segregations within the Voisey’s Bay intrusion reveal systematic variations in Fe-Ni partitioning with sulfide nickel content. In light of the recent experimental results of Brenan and Caciagli (2000), such variation is interpreted as arising from differences in fO2 between the different sulfide bodies. Assuming that each sulfide body became closed to Fe-Ni exchange at the same temperature, the observed fO2 variation is interpreted to reflect intrinsic differences in their oxidation state. Varied-textured troctolite from the Eastern Deeps records the highest oxygen fugacities (10–10.4), with progressively lower fO2 documented in samples from the basal breccia sequence of the Eastern Deeps (10–11.1), Discovery Hill (10–12.0), and Reid Brook zones (10–12.4). In light of both isotopic and petrographic evidence for open-system behavior within the Voisey’s Bay magmatic system, a model of magma reduction involving assimilation of carbon-bearing country rock (i.e., the Tasiuyak gneiss) has been explored. Calculations suggest that the assimilation of graphite (which is present in the more reduced Voisey’s Bay samples) by a relatively oxidized parental magma at high temperature, then closure at lower temperature, can account for both the absolute values and variation in fO2 within the Voisey’s Bay sulfide bodies. Consideration of the fO2 exhibited by other olivine- + sulfide-saturated intrusive suites reveals a range from relatively oxidized, as typified by most oceanic basalts, to the relatively reduced native iron-bearing dike from Disko Island (Greenland). As in the case for Voisey’s Bay, intrusive and extrusive samples that yielded relatively low fO2 also show clear evidence for assimilation of a reducing agent.