The dissolution process of chalcopyrite in the presence of mesophilic microorganisms was investigated by electrochemical measurements. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis, accompanied by leaching experiments. Results proved that the presence of Acidithiobacillus ferrooxidans enhanced the initial reduction of chalcopyrite to intermediate species (Cu2S), thus promoting the dissolution of chalcopyrite in the initial stage of bioleaching. However, chalcopyrite tended to be directly oxidized to polysulfide (S-n(2-)) and CuS in the later stage of bioleaching when redox potential was higher than 0.5 V (vs. Ag/AgCl), the formed polysulfide and jarosite can be responsible for the passivation of chalcopyrite in the later stage of bioleaching by A. ferrooxidans. On the contrary, Chalcopyrite was mainly directly oxidized to polysulfide and CuS in the presence of sterile 91( medium or Acidithiobacillus thiooxidans, and the initial reduction reaction was still the rate-limiting step, thus resulting in low copper extraction rate. (C) 2014 Elsevier Ltd. All rights reserved.