The pyritic sulphur removal from coal and pyrite by Thiobacillus ferrooxidans was studied in a batch reactor. Microbial oxidation of ferrous iron to the ferric form, the central step in the biodesulphurization process was found to be affected by the substrate and product concentration. Direct microbial oxidation of pyrite was dominant during the exponential phase, while indirect electrochemical oxidation was observed at the stationary phase of the growth. The effect of various parameters, such as pulp density, ferrous and ferric iron concentrations on the rate of biodesulphurization was studied. The rate of pyritic sulphur removal was retarded at higher concentrations of ferrous and ferric iron. Therefore, during the process, the concentrations of Fe(II) and Fe(III) iron in the bioreaction mixture need to be controlled to maintain high rates of pyritic sulphur removal.