Sulfur isotope analyses of tars from bituminous sand, crude oils, and solid hydrocarbons from the Uinta Basin (Tertiary) Utah show wide variations, but all are heavier than the troilite sulfur isotopic standard. The data range from 2.7 to 30.3% and sulfur contents are low, usually less than one percent. Crude oils and tars from rocks of Mesozoic and Paleozoic age show S34S values within plus or minus 5% of the meteorite standard and exhibit sulfur contents in excess of one percent. The positive (enriched in S-34) S34S values reflect the progressive enrichment of sulfur-34 in waters and sediments of Tertiary Lake Uinta that arose from bacterial reduction of sulfate and loss of hydrogen sulfide that had been enriched in sulfur-32.