The bioleaching of pyrite has been found to occur via an indirect mechansim. Ferric iron leaches the pyrite, and is reduced to ferrous iron. Bacteria such as Thiobacillus ferrooxidans oxidise the ferrous iron to ferric iron, thus maintaining a high redox potential. The effect of the redox potential on the ferric leach rate was investigated by developing an experimental technique where dynamic redox potential measurements were used to study the kinetics of the sub-process. The ferric leach rate of pyrite was found to be of the order of 3 x 10(-7) moles pyrite per mole pyrite per second, which is of the same order of magnitude as rates reported for the bioleaching of pyrite over similar ranges of redox potential. The rate decreased as the redox potential decreased, in what appeared to be a Butler-Volmer-like manner. This, along with the observation that there was no significant effect of the total iron concentration, suggested the likelihood of an electrochemical mechanism being operative, with charge transfer at the pyrite surface being rate limiting.