In this paper, the results of a study of the leaching of pyrite in the presence and absence of bacteria at the same solution conditions are summarised. These results indicate that the leaching of pyrite occurs at a higher rate in the presence of bacteria than in the absence of bacteria. Analysis of the rate of reaction as a function of concentration indicates that the order of reaction with respect to ferric ions is the same in the presence and the absence of bacteria. However, the order of reaction with respect to Hq is y0.5 in the absence of bacteria, and y0.39 in the presence of bacteria. The results of a study of the mixed potential of pyrite are also presented. These results indicate that the mixed potential of pyrite decreases with time in the presence of bacteria, while it is constant in the absence of bacteria. A detailed mechanism of the leaching of pyrite in ferric sulphate solutions is presented, and this theory is used to interpret the results. It is argued that the effect of the bacteria on the rate of leaching and on the mixed potential is not consistent with the direct contact mechanism of bacteria leaching. Instead, it is shown that the results can be explained by an increase in the pH at the mineral surface as a result of bacterial activity. This means that the increase in the leaching rate is a result of the indirect contact mechanism. q 2001 Published by Elsevier Science B.V.