Published models apply particle size to predict bioleaching reactor performance. In this study mathematical models predicting the overall reaction rate of a bioleach reactor system are developed to establish which particle parameters are necessary to describe reactor performance. Since the intrinsic leaching rate is surface area dependent, the inlet particle size and residence time are model inputs. Three models, distinguished by the incorporation of age and/or size distributions, were developed from first principles through a segregation approach. Explicit dependence on the age distribution, allows the model extension to unsteady state behavior and imperfect mixing. The models are validated against pilot plant data for pyrite-arsenopyrite concentrates obtained from the Fairview Mine in South Africa. Simulation results indicate that only a model that considers both the particle age and size distribution is in good agreement with the data. (C) 2008 American Institute of Chemical Engineers.