Extracellular polymeric substances seem to play a pivotal role in biocorrosion and bioleaching. For improved control of both processes, the structure and function of extracellular polymeric substances from the relevant bacteria are of crucial importance. The research focussed on Acidithiobacillus ferrooxidans and the compn. of its extracellular polymeric substances for a further deduction of their function. The extracellular polymeric substances of Acidithiobacillus ferrooxidans consist mainly of neutral sugars and lipids. They have the functions to (1) mediate attachment to a metal sulfide surface (2) conc. ferric ions by complexation through uronic acids or other residues at the mineral surfaces and thus allowing for an oxidative attack on the sulfide. Consequently, dissoln. corrosion of the metal sulfide is enhanced and may result in a 20-100 fold acceleration of the bioprocess over the chem. process. Expts. were performed to elucidate the importance of the ferric ions complexed by extracellular polymeric substances for strain specific differences in the oxidative activity for pyrite. A large amt. of ferric ions in their EPS causes a high oxidn. activity.