Biological oxidation of ferrous sulphate by Thiobacillus ferrooxidans has proved to be a significant step in the bioleaching of sulphide minerals and treatment of acid mine drainage. The same bioreaction also has beneficial applications in the desulphurization of coal and removal of hydrogen sulphide from gaseous effluents. Owing to the numerous potential industrial applications, the process of biocatalytic oxidation of ferrous iron has been studied extensively over the years. In the present article different aspects of this biological reaction from both a microbiological and engineering point of view are discussed and an overview of the current knowledge with respect to T. ferrooxidans and the process it catalyses is provided. © 1998 Elsevier Science S.A. All rights reserved.