Extracellular polymeric substances seem to play a pivotal role in bioleaching of metals. For better control, the structure and function of extracellular polymeric substances derived from leaching bacteria are of crucial importance. The research focussed on Acidithibacillus ferrooxidans, 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 (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 allow attack of the sulfide. Consequently, dissoln. of the metal sulfide is enhanced and may result in an acceleration of 20-100 fold 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 oxidative activity for pyrite. 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 new insight into the function of extracellular polymeric substances and consequent advantage conferred to bacteria involved in bioleaching.