The kinetics and mechanism of pentlandite leaching in aerated acidic chloride/sulfate solutions were studied using electrochemical and chemical techniques. The electrochemical experiments showed that pentlandite experiences passivation at low overpotentials. These results were consistent with the leaching studies which showed that pentlandite leaching proceeds at a low rate. However, the addition of chloride ions was found to increase the leaching rate, and it is believed that the role of chloride is to allow the formation of a more porous sulfur product. Both temperature and oxygen partial pressure had a substantial effect on the reaction rate, with optimum leaching kinetics being obtained at 85 degrees C and an oxygen pressure of 1 arm. Under such conditions, 96% nickel extraction could be achieved during leaching in the presence of chloride ions. (C) 2000 Elsevier Science B.V. All rights reserved.