Several different microorganisms have been suggested for coal desulfurization. In the present investigation, the thermophilic archaea Acidianus brierleyi (DSM 1651), Sulfolobus acidocaldarius (DSM 639) and Sulfolobus solfataricus (DSM 1616) were compared with the mesophilic bacterium Thiobacillus ferrooxidans (DSM 583) concerning their capability of removing sulfur from coal. The desulfurization rate as well as the amount of sulfur removed by the microorganisms was studied. Two of the investigated microorganisms, Thiobacillus ferrooxidans and Acidianus brierleyi, were capable of oxidizing pure pyrite as well as oxidizing sulfur in coal. A kinetic analysis was performed assuming first order reactions. The rate constant for oxidation of pure pyrite by A. brierleyi was observed to be higher than for T. ferrooxidans, The values of the rate constants for sulfur removal from coal were comparable for the two microorganisms, but were higher than for oxidation of pure pyrite.