This paper describes kinetic study of biological ferrous sulphate oxidation by Thiobacillus ferrooxidans iron-oxidising bacteria in submerged culture and immobilised in nickel alloy fibre as matrix support. In this way, two types of bioreactors has been used: a continuous stirred tank reactor (CSTR) for free cells and a packed bed bioreactor for immobilised biomass. A mathematical expression has been developed to explain kinetic behaviour of micro-organism in bioreactors as function of the main process parameters. Model predictions of ferrous iron oxidation rate were found closely to experimental data and provide high coefficient. Comparison between oxidation rates for two bioreactors showed that process with a biofilm reactor is more stable than the bioreactor with free immobilised biomass. (C) 2002 Elsevier Science Ltd. All rights reserved.