1) Institute of Earth Sciences, Hebrew University, Jerusalem 91904, Israel, 2) US Geological Survey, Denver, CO 80225, USA, 3) EOG Resources, Denver, CO 80202, USA, 4) Biomarker Technologies, Rohnert Park, CA 94928, USA, 5) ExxonMobil Upstream Research Company, Houston, TX 77389, USA Geochimica et Cosmochimica Acta (2015), V167, pp144–161, doi: 10.1016/j.gca.2015.07.008 Compound-specific sulfur isotope analysis of thiadiamondoids of oils from the Smackover Formation, USA Zvi Gvirtzman (1), Ward Said-Ahmad (1), Geoffrey S. Ellis (2), Ronald J. Hill (3), J. Michael Moldowan (4), Zhibin Wei (5), Alon Amrani (1) In this study, we analyzed a suite of 12 Upper Jurassic oil and condensate samples generated from source rocks in the Smackover Formation to perform a systematic study of the sulfur isotope distribution in thiadiamondoids (TD) (one and two cages). For comparison we measured the δ34S composition of benzothiophenes (BTs) and dibenzothiophenes (DBTs). We also conducted pyrolysis experiments with petroleum and model compounds to have an insight into the formation mechanisms of TDs. The δ34S of the TDs varied significantly (ca 30‰) between the different oils depending on the degree of thermochemical sulfate reduction alteration. The results showed that within the same oil, the one-cage TDs were relatively uniform, with 34S enriched values similar to those of the coexisting BTs.