Discrete particle size fractions of ore containing sphalerite and pyrite were bioleached in columns. Both the sphalerite and pyrite entities showed biphasic leach kinetics comprised of an initial colonization or activation phase followed by a steady state phase that was described by shrinking core kinetics. The initial colonization period followed classical bacterial growth kinetics that was described by the logistic equation. The colonization rate constant was a function of the available mineral surface area and followed Monod-type kinetics. The steady rate reaction rate constant was a function of the particle size and fractional mineral content. Bacteria cells attached to sphalerite preferentially over pyrite. During the colonization phase, bacterial reproduction was stimulated more by pyrite than sphalerite even though sphalerite could harbour a greater number of cells. Once colonization was complete and steady rate conditions were achieved, the reaction rate constants for sphalerite were much faster than those of pyrite. (C) 2003 Elsevier Ltd. All rights reserved.