Thiobacillus ferrooxidans oxidises ferrous ions in solution during the bacterial leaching process. In batch experiments that are usually performed to determine the kinetics of the bacterial oxidation of ferrous ions, the concentrations of both the ferrous and ferric ions change as a result of the bacterial activity. This change in concentration makes it difficult to interpret the data from these experiments, particularly if the concentrations vary through reaction regimes where different mechanisms govern the bacterial activity. In this study the concentrations of ferrous and ferric ions are maintained at their initial values by controlling the redox potential of the solution in which the reaction occurs. The redox potential is controlled by reducing the ferric ions produced by the bacteria. In order to achieve this, the bacteria are grown in the cathode compartment of an electrolysis cell. The current to the cell is varied by the controller, depending on whether more or less reduction of ferric ions is required. The current required to reduce the ferric ions in order to maintain the redox potential at a constant value is a measure of the bacterial activity and the rate of bacterial growth. The effect of ferrous, ferric and arsenite concentrations on the rate of bacterial growth are presented. It is shown that growth rate is strongly dependent on the concentration of both the ferric and the arsenite ions in solution.