The effects of chloride ions on chalcopyrite leaching by biologically-produced ferric sulfate solution and on the iron-oxidizing culture were determined. Chloride ions significantly increased chalcopyrite leaching by ferric sulfate at 67degreesC and 87degreesC, but slowed down the leaching at 50degreesC. At 90degreesC, chloride at 5 g dm(-3) (0.25 g Cl- g(-1) concentrate) increased the copper yield from 60 to 100% in approximately 2 weeks. Further increase in Cl-concentration did not affect the leaching. Addition of chloride increased both leaching yields and iron precipitation, which shows that the passivation was not due to iron precipitation. A decreased Ag-potential of 60 mV against an Ag/AgCl reference electrode in the presence of Cl- indicates the accumulation of partially oxidized forms of dissolved sulfur compounds such as thiosulfate and polythionate instead of elemental sulfur and, thus, a decrease in sulfur passivation. A chloride concentration of 5 g dm(-3) did not affect the iron oxidation rate of the iron-oxidizing culture dominated by Leptospirillum ferriphilum. (C) 2004 Society of Chemical Industry.