Processing Ugljevik lignite with mixed anaerobic cultures containing sulfate-reducing bacteria resulted in significant transfer of sulfur from the sulfatic fraction to the organic sulfur fraction. Although desulfurization had been the objective of the experiments, coal retrieved from microbial cultures was up to 12% higher in total sulfur, and up to 20% higher in the organic sulfur fraction, than coal retrieved from sterile controls. Sulfur mass balances suggested that sulfatic sulfur was rinsed away during processing in controls, but in cultures was reduced to sulfide, which reassociated with coal. Sulfide sorption experiments were performed at pH 7.5 using Ugljevik lignite, a North Dakota lignite, and two Illinois Basin coals, IBC-108 and IBC-101. Respective sulfide capacities were: 22.95, 16.85, 11.44 and 7.36 mg g −1. Sulfide sorption correlated with ash contents ( r = + 0.86) and with ash free carbon contents ( r = − 0.95). These results cast doubt on reductive removal as a desulfurization option. However, they support the scientific hypothesis that dissimilatory sulfate reduction contributed to diagenesis of high sulfur coal.