Thiobacillus ferrooxidans is a facultatively aerobic bacteria which catalyses the oxidation of inorganic substrates; in particular mineral sulfides. The mechanism(s) for the oxidation of mineral sulfides is not completely understood. The direct oxidation mechanism involves the attachment of bacteria to the mineral surface. Scanning Electron Microscopy (SEM) was used to investigate the surface erosion of three mineral sulfide samples by the attachment of Thiobacillus ferrooxidans (DSM 583). The mineral samples; Pyrite, a chalcopyrite concentrate (termed Chalconc) and a sample containing arsenopyrite and loellingite (FeAs2) (termed Arsenoconc) were all mounted in resin blocks and following the addition of T.ferrooxidans the particles exhibited varying degrees of surface erosion. Erosion patterns on the surface of minerals from the chalconc and arsenoconc samples appeared to follow the crystallographic structure of the mineral species. However, no apparent erosion patterns were observed on the pyrite sample. In addition, elemental sulfur was found deposited on the surfaces of each mineral sample. Chalopyrite in the Chalconc sample exhibited preferential erosion compared with the pyrite and indicated the electrochemical nobility of pyrite. In addition, during the oxidation of the Arsenoconc sample, loellingite was seen to be have been significantly more oxidised than the arsenopyrite. The observations from the SEM suggested a greater involvement of the indirect oxidation mechanism which utilises the oxidant ferric iron than direct bacterial attachment. (C) 2000 Elsevier Science Ltd. All rights reserved.