Nowadays there is an increasing interest for applying iron reducing bacteria to hydrometallurgical processes. Although most isolated bacteria grow at circumneutral pH, Acidiphilium cryptum is able to reduce iron oxides in acidic environments and promote its dissolution. In this study, the reductive dissolution of magnetite and jarosite by A. cryptum JF-5 was tested in two culture media, over a period of 38 days with 1% pulp density. In a medium without oxalic acid, bacteria were crucial to increase the magnetite and jarosite dissolution from 0.2 and 0.3% to 20.4 and 2.5%, respectively. In a medium containing oxalic acid, the abiotic dissolution of magnetite and jarosite (1.6 and 2.0%, respectively) was clearly surpassed in the inoculated tests (to 26.9 and 10.4%, respectively). The pH control was essential to attain a significant degree of magnetite dissolution. A stoichiometric ratio between sulfuric acid consumed and iron dissolved was obtained for both solids tested. Finally, magnetic stirring tests pointed out that the dissolution process was kinetically controlled by the solid surface.