The main objective of this research was to evaluate the potential of electrochemical bioleaching to extract copper from pyritic chalcopyrite concentrates. Bacterial and chemical (uninoculated) shake flask leaching of Sarcheshmeh copper concentrate at 15% (w/v) pulp density, 150 rpm and stirred tank electrochemical bioleaching of the concentrate at ORP (oxidation reduction potential) ranging from 400 to 430 mV (vs. Ag/AgCl), 20% pulp density and 600 rpm were conducted with and without pyrite addition. A mixed culture of moderately thermophilic microorganisms was used in all bioleaching experiments at an initial pH of 1.5, 50 degrees C, Norris nutrient medium and 0.02% (w/w) yeast extract addition. The results of leaching experiments in shake flasks showed that the addition of pyrite to the concentrate significantly increased the efficiency of copper extraction especially in the presence of microorganisms. In electrochemical bioleaching process, both the rate and extent of copper extraction were selectively (with respect to iron) enhanced in the pyritic copper concentrate in which about 90% copper recovery was achieved from the concentrate after 10 days. Analyses of optical microscopy and SEM/EDS revealed that pyrite remained unaffected in the electro-biochemical system while chalcopyrite was preferentially dissolved. It can also be concluded that at low levels of solution ORP, pyrite remains inert, which acts as a cathode site relative to chalcopyrite and other copper sulfide minerals (galvanic interaction) leading to enhance the anodic dissolution of the copper bearing minerals. Electrochemical system regulates the ratio of ferric to ferrous iron at an optimum level where the dissolution rate of chalcopyrite is maximum. Sulfur oxidizer microorganisms intensify the galvanic interactions and the rate of electron transfer among sulfides by removing the insulating sulfur product. (C) 2012 Elsevier Ltd. All rights reserved.