Accumulation of elemental sulfur during pyrite oxidn. lowers the efficiency of coal desulfurization and bioleaching. In the case of pyrite bioleaching by Leptospirillum ferrooxidans, an iron(II)-ion-oxidizing organism without sulfur-oxidizing capacity, from the pyritic sulfur moiety about 10% elemental sulfur, 2% pentathionate, and 1% tetrathionate accumulated by a recently described cyclic pyrite oxidn. mechanism. In the case of pure cultures of Thiobacillus ferrooxidans and mixed cultures of L. ferrooxidans and T. thiooxidans, pyrite was nearly completely oxidized to sulfate because of the capacity of these cultures to oxidize both iron(II) ions and sulfur compds. Pyrite oxidn. in acidic solns., mediated chem. by iron(III) ion, resulted in an accumulation of similar amts. of sulfur compds. as obtained with L. ferrooxidans. Changes of pH to values below 2 or in the iron ion concn. are not decisive for diverting the flux of sulfur compds. The literature on pyrite bioleaching is in agreement with the findings indicating that the chem. of direct and indirect pyrite leaching is identical.