Previous studies have shown that the different preferences of thermophiles to oxidize S(0) or Fe(2+) is reflected by different [Fe(3+)]/[Fe(2+)] levels in solution. In those studies it was concluded that [Fe(3+)]/[Fe(2+)] governs the thermophilic bioleaching of chalcopyrite rather than temperature or pH. Therefore, the proposed model is mainly based on the finding that thermophilic bioleaching of chalcopyrite is governed by [Fe(3+)]/[Fe(2+)] that result from the activity of thermophiles. A direct interaction between chalcopyrite and thermophiles is neglected because it has been reported that this is not a general behavior for all thermophiles. The case of constant temperature, initial pH 1.5-2.5, and chalcopyrite concentrates is considered. The main assumption is that chalcopyrite can be anodically oxidized or cathodically reduced depending on [Fe(3+)]/[Fe(2+)] in solution. When chalcopyrite is oxidized at high [Fe(3+)]/[Fe(2+)] levels, Cu(2+) is formed directly at low rates: CuFeS(2) + 4Fe(3+) --> Cu(2+) + 5Fe(2+) + S(0). Whereas, when chalcopyrite is reduced at low [Fe(3+)]/[Fe(2+)] levels, an intermediate (Cu(2)S) is formed at higher rates: CuFeS(2) + Fe(2+) + Cu(2+) + 2H(+) --> Cu(2)S + 2Fe(3+) + H(2)S. Because the oxidation of Cu(2)S is relatively fast: Cu(2)S + 4Fe(3+) --> 2Cu(2+) + S(0) + 4Fe(2+), its accumulation is assumed to be negligible. To take into account the possibility of chalcopyrite being oxidized or reduced depending on [Fe(3+)]/[Fe(2+)] in solution, the principle of mixed potentials is used. The model is validated by comparing the calculated and measured values of copper extraction, total iron in solution, and pH. (C) 2009 Elsevier Ltd. All rights reserved.