Abstract Oxidative leaching of chalcopyrite with dissolved oxygen and/or with ferric ions is promoted by high concentrations of ferrous ions in sulfuric acid solutions containing cupric ions. This paper proposes a reaction model to interpret this phenomenon and the thermodynamics of the leaching is discussed. The model considers the leaching to take place in two steps: (1) reduction of chalcopyrite to Cu 2S by ferrous ions in the presence of cupric ions and (2) oxidation of the Cu 2S to cupric ions and elemental sulfur by dissolved oxygen and/or by ferric ions. The intermediate Cu 2S is more amenable to oxidation than chalcopyrite, causing enhanced copper extraction. The model predicts that the formation of intermediate Cu 2S and ferrous-promoted chalcopyrite leaching occur when the redox potential of the solution is below a critical potential that is a function of the ferrous and cupric ion concentrations. To confirm this, flask-shaking leaching experiments were carried out with 0.1 mol dm −3 sulfuric acid solutions containing known concentrations of ferrous, ferric, and cupric ions at 303 K in air. The results agreed well with the predictions, i.e. copper extraction was enhanced at solution potentials below the critical potential predicted with the model.