Extracellular polymeric substances seem to play a pivotal role in bioleaching 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 from leaching bacteria are of crucial importance. The research focussed on Acidithiobacillus ferrooxidans, the compn. of its extracellular polymeric substances and further deduction of their function. The extracellular polymeric substances of Acidithiobacillus ferrooxidans consist mainly of neutral sugars and lipids. The functions of the extracellular polymeric substances of this leaching bacterium seem to be to (1) mediate attachment to a metal sulfide surface (2) conc. iron(III) ions by complexation through uronic acids or other residues at the mineral surface and, thus, allowing for an oxidative attack on the sulfide. The consequence is the enhancement of metal sulfide, dissoln. which may result in an acceleration of 20-100 fold over that of chem. leaching. Expts. were performed to elucidate the importance of the iron(III)ions complexed by extracellular polymeric substances for strain specific differences in oxidative activity for pyrite dissoln. The strains of Acidithiobacillus ferrooxidans with a high amt. of iron(III) ions in their extracellular polymeric substances possess a higher oxidn. activity than those with less iron(III) ions. These data provide insight into the function and consequently the advantage extracellular polymeric substances confer to bacteria involved in bioleaching.