An understanding of the evolution of sulfur compounds in oils may allow petroleum geochemists to direct exploration toward low-sulfur crudes. Knowing the types and chemistry of sulfur compounds is important because (1) some degrade refining catalysts, (2) others produce excessive corrosion, and (3) clean-air requirements necessitate reduction of overall sulfur levels. Seventy-eight crudes from many of the world's major fields which represent reservoirs of Cambrian to Pliocene age were analyzed and then grouped into three categories on the basis of the relative amounts and chemical stability of different sulfur-compound types. These groupings are consistent with geochemical concepts of thermal maturation and biodegradation. One group has abundant unstable sulfur compounds uch as nonthiophenic sulfides and benzothiophenes; we call this immature. Another contains more of the stable benzothiophenes and we call it mature, whereas the third has intermediate distributions which we term altered. Most of the sulfur (particularly in high-sulfur oils) is introduced secondarily by reaction of hydrocarbons with sulfur or hydrogen sulfide (both generated by the microbial utilization of hydrocarbons and sulfates in rocks). This forms a crude rich in nonthiophenic sulfides and occurs primarily during the early diagenesis of the organic matter in the source; with increasing maturation, sulfur is removed preferentially, forming a mature oil with relatively more thiophenes. At shallow depths, alteration produces crudes with more nonthiophenic sulfides and benzothiophenes. nother form of "altered" oils is adjacent to zones of organic metamorphism where H₂S reacts with oils to produce extremely corrosive mercaptan-rich crudes and condensates. These concepts can be applied to the specifics of petroleum-exploration geochemistry.