The results of recent research have shown that the bioleaching of sulfide minerals occurs via a two-step mechanism. In this mechanism, the sulfide mineral is chemically oxidized by the ferric-iron in the bioleaching liquor. The ferrous-iron produced is subsequently oxidized to ferric-iron by the microorganism. Further research has shown that the rates of both the ferric leaching and ferrous-iron oxidation are governed by the ferric/ferrous-iron ratio (i.e., the redox potential). During the steady-state operation of a bioleach reactor, the rate of iron turnover between the chemical ferric leaching of the mineral and the bacterial oxidation of the ferrous-iron will define the rate and the redox potential at which the system will operate. The balance between the two rates will in turn depend on the species used, the microbial concentration, the residence time employed, the nature of the sulfide mineral being leached, and its active surface area. The model described proposes that the residence time and microbial species present determine the microbial growth rate, which in turn determines the redox potential in the bioleach liquor. The redox potential of the solution, in turn, determines the degree of leaching of the mineral; that is, conversion in the bioleach reactor.