Measurements of stable isotopic variations (sulfur, carbon, and oxygen), supplemented by chemical, mineralogical, and petrographic analyses, were used to investigate the processes responsible for the incorporation of sulfur in coal and the development of permineralized peat zones (coal balls). Sulfur isotopic results indicate that there are two principal sources in Pennsylvania coals from the Illinois Basin: (1) primary sulfur assimilated by plants (low-sulfur coal), and (2) secondary sulfur produced by reactions involving h2s that was produced by the anaerobic reduction of sulfate in seawater that permeated the peat during postdepositional marine transgressions (the dominant source of sulfur in high-sulfur coals). Sulfur isotopic compositions of low-sulfur coals in several Tertiary basins of Wyoming are consistent with a plant sulfur source. However, the inverse correlation between content and isotopic composition of sulfur observed in coals from the Powder River Basin suggest some secondary additions to these low-sulfur coals. Carbon and oxygen isotopic compositions of calcareous coal balls from North America and Europe suggest that the carbonate was derived from variable proportions of oxidized plant matter and inorganic carbonate, and the carbonate minerals were precipitated from fresh-to-brackish waters.