Introduction Sulfur is a widely distributed impurity in crude oils.l * It is found in the form of sulfides@ mercaptans@ thiophenes@ and complex compounds of high molecular weight. The concentrations arid distributions of sulfur in crudes vary significantly between various source fields. Inasmuch as a given refinery may process up to 50 different crudes in a year@2 the refining process must be capable of handling the maximum amount of sulfur in the feed. During refining@ sulfur compounds may be emitted in the form of hydrogen sulfide@ sulfur dioxide@ sulfur trioxide@ sulfuric acid mist@ organic sulfides@ mercaptans@ and miscellaneous complex compounds having foul odors. These emissions must be controlled to minimize the effect of the refinery on the surroundings and to comply with the applicable air pollution control regulations. This chapter discusses the nature of the various sulfur compounds that may be emitted@ their sources in the refinery@ and possible methods of control. A brief discussion of air pollution control regulations is also provided. *REFERENCES on pages 8-7 and 8-8.