Sulfur isotope ratios of sulfide ores and their host rocks, and the overlying main mass sequence of norites, quartz gabbros, and granophyres from the Sudbury Igneous Complex have been analyzed to establish the extent of isotopic heterogeneity of the Complex and the impact melt sheet from which it was produced. Noritic rocks located above the sublayer in the lower portion of the main mass of the Sudbury Igneous Complex are characterized by uniform sulfur isotope ratios with δ34S values of 2.59 ± 0.48‰. Rocks from the granophyric portion of the main mass are characterized by elevated and variable δ34S values from 4.4 to 14.1‰. The more 34S-enriched S in the granophyre was produced by secondary sulfide precipitation related to hydrothermal alteration systems developed in the upper altered granophyric part of the main mass as it cooled. Footwall and contact-style mineralization from the sublayer show variable δ34S values from 0.2 to 4.3‰. South Range mineralization is characterized by low ratios from 0.2 to 2.5‰. North and Northeast Range mineralization exhibits locally elevated but variable δ34S values; for example mineralization from Whistle mine varies from 3.1 to 4.3‰, whereas mineralization from Victor varies between 2.4 and 2.9‰. There is typically only a small difference in S isotope ratio of contact and proximal footwall mineralization. Country rocks in the area of the Sudbury Igneous Complex are also characterized by a wide range in δ34S values and the sulfur isotope homogeneity of the noritic rocks can be explained by mixing of sulfur from several potential sources. However, the variability present between deposits at the lower contact of the Complex requires that country rocks of locally variable sulfur isotope composition interacted with magma at the base of the cooling melt sheet. There is no evidence that deposit size correlates with δ34S value; variations between deposits may involve local S isotope exchange reactions between the country rock and the melt sheet as the melt digested the footwall. This study illustrates the application of S isotope data in constraining the scale of convective mixing and contamination in the largest known terrestrial melt sheet.