This work investigates the bioleaching of a complex nickel-iron concentrate (pentlandite, pyrrhotite, and minor amounts of chalcopyrite) using acidophile iron-oxidizing bacteria. It aims to improve the understanding of the mechanism of bacterial action on nickel sulphide bioleaching. The effects of the external addition of Fe(II) and the mineralogical assembly on the extraction of nickel are evaluated. A high nickel extraction (around 70%) can be achieved in batch experiments. Moreover, the external addition of iron has not shown any effect on the extraction of the metal, emphasizing the importance of pyrrhotite dissolution in the first step of bioleaching. It was also examined the morphological features of the sulphides as well as the leach residues and reactions products formed during bioleaching. It was noticed that elemental sulphur was produced on pyrrhotite surfaces, which dissolves ahead of pentlandite. A discussion on how galvanic interactions affect the reactivity of sulphide mineral and the formation of bioleaching products is also presented. (C) 2006 Elsevier Ltd. All rights reserved.