Abstract Mass spectrometric measurements have been made of the isotopic composition of sulfur in troilite from meteorites and in sulfides from igneous rocks and genetically associated mineral deposits of the Triassic Newark group, in eastern United States. Some of the intrusives sampled are strongly differentiated. The igneous rocks are tholeiitic and presumably originated in the upper mantle. Considering the probable isotopic inhomogeneity of sulfur in the many rock types underlying the Newark basins, the observed isotopic uniformity of sulfides in normal diabase from intrusives suggests little contamination with crustal sulfur. The following δS 34 per mil values were obtained: meteoritic troilite—average +0.2 ± 0.1; normal diabases from intrusives—average +0.1 ± 0.4; late stage facies from differentiated intrusives—mostly +3 to +8, maximum +11, with pyrite enriched in S 34 relative to coexisting chalcopyrite; specimens from the Cornwall, French Creek, and Dillsburg deposits, formed from emanations from adjacent diabase intrusives,—range from −0.2 to +17, averaging about +7; extrusives—range from −12 to +0.7, averaging about −5. The standard deviation analytical error is ±0.2. Data are calculated relative to Cañon Diablo troilite with an assumed S 32/S 34 ratio of 22.220. From these data, it is concluded that: (1) beneath the Newark basins, during late Triassic time, upper mantle sulfur was isotopically similar to meteoritic sulfur, (2) isotopic fractionation can occur during crystallization of tholeiitic magma, resulting in a slight enrichment of S 34 in late stage sulfides, (3) fractionation may continue into hydrothermal stages, causing an enrichment of S 34 in magnetic hydrothermal sulfides and (4) by some mechanism of isotopic fractionation associated with crystallization at the earth's surface, sulfides in Newark extrusives are slightly enriched in S 32 relative to the magma source.