Sulfur occurs in multiple mineral forms in coals, and its fate in coal combustion is still not well understood. The sulfur isotopic composition of coal from two coal mines in Indiana and fly ash from two power plants that use these coals were studied using geological and geochemical methods. The two coal beds are Middle Pennsylvanian in age; one seam is the low-sulfur (< 1%) Danville Coal Member of the Dugger Formation and the other is the high-sulfur (> 5%) Springfield Coal Member of the Petersburg Formation. Both seams have ash contents of approximately 11%. Fly-ash samples were collected at various points in the ash-collection system in the two plants. The results show notable difference in delta S-34 for sulfur species within and between the low-sulfur and high-sulfur coal. The delta S-34 values for all sulfur species are exclusively positive in the low-sulfur Danville coal, whereas the delta S-34 values for sulfate, pyritic, and organic sulfur are both positive and negative in the high-sulfur Springfield coal. Each coal exhibits a distinct pattern of stratigraphic variation in sulfur isotopic composition. Overall, the delta S-34 for sulfur species values increase up the section in the low-sulfur Danville coal, whereas they show a decrease up the vertical section in the high-sulfur Springfield coal. Based on the evolution of delta S-34 for sulfur species, it is suggested that there was influence of seawater on peat swamp, with two marine incursions occurring during peat accumulation of the high-sulfur Springfield coal. Therefore, bacterial sulfate reduction played a key role in converting sulfate into hydrogen sulfide, sulfide minerals, and elemental sulfur. The differences in delta S-34 between sulfate sulfur and pyritic sulfur is very small between individual benches of both coals, implying that some oxidation occur-red during deposition or postdeposition. The delta S-34 values for fly ash from the high-sulfur Springfield coal (averaging 9.7 parts per thousand) are greatly enriched in S-34 relative to those in the parent coal (averaging 2.2 parts per thousand). This indicates a fractionation of sulfur isotopes during high-sulfur coal combustion. By contrast, the delta S-34 values for fly-ash samples from the low-sulfur Danville coal average 10.2 parts per thousand, only slightly enriched in S-34 relative to those from the parent coal (average 7.5 parts per thousand). The delta S-34 values for bulk S determined directly from the fly-ash samples show close correspondence with the delta S-34 values for SO4-2 leached from the fly ash in the low-sulfur coal, suggesting that the transition from pyrite to sulfate occurred via high-temperature oxidation during coal combustion. (c) 2007 Elsevier B.V. All rights reserved.