Jarosite precipitation is a very important phenomenon that is observed in many bacterial cultures. In many applications involving Acidithiobacillus ferrooxidans, like coal desulphurization and bioleaching, it is crucial to minimize jarosite formation in order to increase efficiency. The formation of jarosite during the oxidation of ferrous iron by free suspended cells of A. ferrooxidans was studied. The process was studied as a function of time, pH and temperature. The main parameter affecting the jarosite formation was pH. Several experiments yielded results showing oxidation rates as high as 0.181-0.194 g/L It, with low jarosite precipitation of 0.0125-0.0209 g at conditions of pH 1.6-1.7 with an operating temperature of 35 degrees C. (C) 2005 Elsevier Ltd. All rights reserved.