Sphalerite (ZnS), a mineral of particular hydrometallurgical interest, is generally found associated with galena (PbS). As such, the effect of galena on the leaching of sphalerite is of importance, and has been studied here for bacterial and ferric sulphate leaching. During both leaching processes galena was selectively oxidised to anglesite in favour of the dissolution of sphalerite. It is believed that this selective behaviour is due to galvanic interactions between the two minerals, whereby galena is sacrificed and sphalerite is passivated. This theory is consistent with the order of measured rest potential values of both minerals in solution, being 325 mV for galena and 375 mV for sphalerite (versus a standard hydrogen electrode). During bacterial oxidation, sphalerite passivation was observed across a range of mixed sphalerite/galena samples, including a mineral species and four ore specimens of varying grades. From the bacterial oxidation of a mixed mineral species, sphalerite was found to leach in the presence of lead sulphate precipitate, though the presence of this precipitate is believed to have caused diffusion limitation. (C) 2003 Elsevier Science Ltd. All rights reserved.