Extracellular polymeric substances seem to play a pivotal role in biocorrosion of metals and bioleaching, biocorrosion of metal sulfides for the winning of precious metals as well as acid rock drainage. For better control of both processes, the structure and function of extracellular polymeric substances of corrosion-causing or leaching bacteria are of crucial importance. Research focused on the extremophilic bacteria Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans, because of the simplicity and knowledge about the interactions of these bacteria with their substrate and environment. For this purpose, the compn. of the corresponding extracellular polymeric substances and their functions were analyzed. The extracellular polymeric substances of both species consist mainly of neutral sugars and lipids. The functions of the exopolymers seem to be (1) to mediate attachment to a metal sulfide surface (2) to conc. iron(III) ions by complexation through uronic acids or other residues at the mineral surface and allow an oxidative attack on the sulfide. Consequently, dissoln. of the metal sulfide is enhanced and may result in an acceleration of 20-100 fold of the bioleaching process over chem. leaching. Expts. were performed to elucidate the importance of the iron(III) ions complexed by extracellular polymeric substances for strain-specific differences in the oxidative activity for pyrite. Strains of A. ferrooxidans with a large amt. of iron(III) ions in their extracellular polymeric substances possess greater oxidn. activity than those with fewer iron(III) ions. These data provide insight into the function of and consequently the advantages that extracellular polymeric substances provide to bacteria. The role of extracellular polymeric substances for attachment under the conditions of a space station and resulting effects like biofouling, biocorrosion and malodorous gases is discussed.