Acidophilic bacteria found in mining environments are capable of oxidising insoluble sulfide minerals. The use of these bacteria during the oxidation of various mineral sulfides has received significant commercial interest. The attachment of Thiobacillus ferrooxidans and moderately thermophilic bacteria to sulfide minerals was investigated to further understand the mechanism(s) involved in the leaching of sulfide minerals. T. ferrooxidans (DSM 583 and ATCC 23270) and four strains of moderate thermophilic bacteria, Sulfobacillus thermosulfidooxidans, (strain TH1) and Sb.acidophilus (strains THWX, ALV and YTF1) all grown on ferrous iron, sulfur and a chalcopyrite concentrate (termed chalconc) were investigated using 3 sulfide mineral systems; pyrite, a chalcopyrite concentrate (chalconc) and an arsenic containing concentrate (termed arsenoconc). The degree of attachment of all substrate grown cells along with contact angle measurements of both minerals and cells were determined in order to evaluate the effect of the growth substrate and the hydrophobic interactions on the attachment process. The attachment of both the mesophiles and moderate thermophiles were found to be dependant on the type of growth substrate, the substrate concentration and also the type of mineral studied Whilst sulfur-grown T.ferrooxidans (DSM 583) cells exhibited a higher degree of hydrophobicity, both ferrous iron and chalconc-grown cells showed a greater degree of attachment. This suggests that hydrophobic interactions are not principally responsible for the attachment of T.ferrooxidans to mineral sulfides. However, each moderately thermophilic strain exhibited greater attachment to the mineral sulfides when cultured on either the chalconc sample or elemental sulfur and all showed greater adhesion to the pyrite and arsenoconc samples than to the chalconc sample. Separate shake flask leaching of the sulfide samples T.ferrooxidans and Sb.thermosulfidooxidans (TH1) respectively in conjunction with the results of the attachment studies suggested that the leaching of mineral sulfides was a combination of both the direct and indirect mechanisms. (C) 2000