In the case of pyrite bioleaching by Leptospirillum ferrooxidans, an organism without sulfur-oxidizing capacity, besides the prodn. of tetra- and pentathionite, a considerable accumulation of elemental sulfur occurred. A similar result was obtained from chem. oxidn. assays with acidic, sterile iron(III) ion-contg. solns. In the case of Thiobacillus ferrooxidans, only slight amts. of elemental sulfur were detectable because of the organism's capacity to oxidize sulfur compds. In the course of oxidative, chem. pyrite degrdn. under alk. conditions, the accumulation of tetrathionate, trithionate, and thiosulfate occurred. The data indicate that thiosulfate, trithionate, tetrathionate, and disulfane-monosulfonic acid are key intermediate sulfur compds. in oxidative pyrite degrdn. A novel (cyclic) leaching mechanism is proposed which basically is indirect.