The bioleaching of bornite with mixed moderately thermophilic culture at 50 °C was investigated. The intermediary species formed during the leaching of bornite were characterized by XRD and XPS. In addition, the evolution of Cu-state during leaching of bornite was further studied by applying φh–pH diagram and cyclic voltammetry. The results showed that the bornite was more likely to be leached at high redox potential. Furthermore, the intermediary sulfides, such as isocubanite, covellite, chalcopyrite,disulfide, and polysulfide, were formed in the course of bornite dissolution. The Cu 2p photoelectron spectrum revealed that the valence of copper in bornite and intermediary sulfide formed in the dissolution of bornite is +1. The bornite and chalcopyrite can be converted into each other, and both can be further converted to covellite and/or chalcocite.