The delta 34 S values of massive pyrite, disseminated pyrite, and organic sulfur from five major Illinois Basin coals have been determined. Disseminated pyritic and organic sulfur were chemically removed from banded ingredients and whole coal samples prior to isotopic analysis. Quantitative extraction of sulfur from vertical coal columns indicated that disseminated pyrite content varies greatly when compared to that of organic sulfur. Individual coal samples were classified as high or low sulfur based on organic sulfur content. High sulfur coals (>0.8% organic sulfur by dry weight) exhibited a wide range of delta values for organic sulfur (--8 to +15ppm), whereas low sulfur coals showed a narrow range (+5 to +13ppm). The isotopic ratio of disseminated pyritic sulfur exhibits large variations when compared to organic sulfur. The delta 34 S for organic sulfur in all three banded ingredients from the same coal sample are very similar. The delta 34 S and content of organic sulfur from vitrain and attrital coal in a high and low sulfur coal exhibit a linear trend of increasingly heavier delta values with decreasing sulfur content. This suggests the mixing of two isotopically distinct sources in the organic sulfur.Whole coal organic sulfur and massive pyrite from the same column exhibit little variation in delta 34 S (generally <10ppm) in comparison to their overall regional range, (24ppm and 40ppm, respectively). A linear relationship exists between organic and massive pyritic sulfur (delta 34 S mp = +1.1 delta 34 S or --4.8ppm) for high sulfur coals. This and additional evidence has been employed to construct a model describing the source and pathways of sulfur incorporation in high and low sulfur coals. Calculations based on this model indicate that an average of about 63 percent of the organic sulfur in high sulfur coals was contributed by bacterially reduced sulfate, whereas only about 13 percent was contributed by the same source in low sulfur coals. The other major source of organic sulfur in coals was plant-assimilated sulfur.