An investigation into the sulphur oxidation of iron grown Thiobacillus ferrooxidans is reported. A new mechanism for sulphur catabolism in this obligate acidophile, involving the initial reduction of the substrate, is proposed. Evidence to support substrate reduction comes from the evolution of H2S in both aerobic and anaerobic conditions (O2 and Fe3+ acting as terminal electron acceptors, respectively) and a accurate trapping method of detection is described. Levels of sulphur reduction are such that a role in the full catabolism of sulphur can be envisaged. In addition, red selenium reduction was investigated and the bioenergetic implications for both substrates are discussed.