The chem. of S leaching from pyrite (FeS) was studied using various litho-trophic Fe(II) oxidizing bacteria. The bacterial strains were Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, and an archae bacterium of the genus Sulfolobus/Acidianus. The formation of S compds. were measured by HPLC and diode array detection. The degrdn. of FeS proceeded via polythionates in a novel cyclic pathway, and elemental S was only a side-product. At neutral pH, polythionates accumulated in high amts. At acidic pH mainly sulfate along with some elemental S and polythionates were formed. To test the validity of the cyclic pyrite degrdn. mechanism for metal sulfides besides pyrite, the chem. oxidn. of hauerite (MnS2), sphalerite (ZnS), and galena (PbS) by Fe(III) ions in acidic soln. was studied. Degrdn. of ZnS and PbS did not proceed via polythionates, but high amts. of elemental S accumulated at acidic pH, which indicated a different mechanism. This can be explained by the different crystal structure and soly. of the metal sulfides.