Scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) were used to follow the surface changes occurring in galena when the bioleaching medium contained Ag(I), Hg(II) and Bi(III) ions. The results showed that the catalyst ions Ag(I), Hg(II) and Bi(III) are incorporated into the surface irrespectively of the presence of bacteria, and different reactions take place on the surface of galena depending on the cation added to the leaching medium. When the bioleaching process takes place without catalyst or with Bi(III) the galena is transformed into PbSO4 and the growth of oxidation products is characterized by needles. Ag(I) and Hg(II) ions form a layer on the galena surface, which prevents the transformation of such surface into PbSO4. The layer formed in this process can be like a silver sulphide and a mercury sulphide depending on the ion species. Microorganisms oxidize the surface sulphur of this layer and thus decrease the S/Ag and S/Hg ratios at the surface.