In the biooxidation of minerals, cells of Thiobacillus are distributed between the surface of the particles and the liquid. This work quantifies the kinetics of attachment, the equilibrium between attached and suspended cells, and the influence of ferric ion and particle size on this phenomenon. The attachment kinetics were fast, the equilibrium was reached in 10 to 30 minutes, depending on the initial population. The equilibrium curves showed three distinct phases, and the first two could be modelled by Langmuir equations. The maximum concentration of attached cells increases with the addition of ferric ions and decreases with particle size.