The use of off-gas analysis and redox potential measurement has shown that bioleaching involves at least three important sub-processes. The primary attack of the sulphide mineral is a chemical ferric leach. The role of the bacteria is to convert the iron from the ferrous to the ferric form, thereby maintaining a high redox potential. The kinetics of bacterial ferrous iron oxidation by Thiobacillus ferrooxidans and a Leptospirillum-like bacterium, and the chemical ferric leach kinetics of pyrite have both been described as functions of the ferric/ferrous-iron ratio. Thus, the chemical ferric leach of the mineral and the bacterial oxidation of the ferrous iron are linked by the redox potential, and are in equilibrium when the rate of iron turnover between the mineral and the bacteria is balanced. These rate equations have been used to predict the steady state redox potential and sulfide mineral conversion in a continuous bioleach reactor. The model successfully predicts laboratory data and is being tested against data from pilot-plant and full-scale bioleach systems. Furthermore, the model predicts which bacterial species will predominate and which mineral will be preferentially leached under specific operating conditions. Enzyme restriction analysis has shown that in pyrite-arsenopyrite bioleach reactors the dominant iron oxidizer is L. ferrooxidans, which is in agreement with the predictions of the model.