Research on the chem. of pyrite bioleaching demonstrated-contradictory to the text book theory on direct or indirect leaching mechanisms-that only the indirect one is functioning. Cells of Thiobacillus ferrooxidans and Leptospirillum ferrooxidans primarily attach to the surface of pyrite particles by electrostatic interactions caused by iron(III) ion contg. extracellular polymeric compds. If sufficient free iron(III) ions are available (0.2 g/l) pyrite degrdn. starts. The first degrdn. products are thiosulfate and iron(II) ions. Thiosulfate will be rapidly oxidized by iron(III) ions to tetrathionate. Tetrathionate adsorbs to the pyrite surface and is hydrolized to sulfane-monosulfonic acid and sulfate plus one proton. From the sulfane-monosulfonic acid several polythionates and elemental sulfur may arise. Consequently, the function of the leaching bacteria is \"only\" the maintenance of a high redox potential by keeping the iron(III) ions mainly in the oxidized state to optimize the indirect attack on the metal sulfide.