Pyrite and arsenopyrite concentrates were oxidized during growth of a variety of acidophilic microorganisms over a wide temperature range. A mesophilic culture, comprising Thiobacillus ferrooxidans and Leptospirillum ferrooxidans as the principal iron-oxidizing bacteria, was used at 30 degrees C; Sulfobacillus thermosulfidooxidans was active in pure and mixed cultures at 48 degrees C; and Sulfolobus-like, thermoacidophilic archaea were grown at up to 84 degrees C. At low mineral concentrations, the rate of pyrite/arsenopyrite dissolution was proportional to temperature. However, the use of elevated temperatures to increase rates of mineral processing over those obtainable with mesophiles appeared to be most practicable with moderately thermophilic bacteria because growth of Sulfolobus strain BC was inhibited at higher mineral concentrations. Two aspects of higher temperature bioleaching were emphasized: the unique capacity of Sulfolobus-like archaea for extensive chalcopyrite oxidation; and the requirement for progress in leaching at high mineral concentrations before the potential of these organisms can be realised in process development.