The kinetics of ferrous iron oxidation by Leptospirillum ferriphilum (L. ferriphilum) dominated culture was studied in the concentration range of 0.1-20 g Fe2+/L and the effect of ferric iron (0-60 g Fe3+/L) on Fe2+ oxidation was investigated at pH below one. Denaturing gradient gel electrophoresis of PCR amplified 16S rRNA genes followed by partial sequencing confirmed that the bacterial community was dominated by L. ferriphilum. In batch assays, Fe2+ oxidation started without lag phase and the oxidation was completed within 1 to 60 h depending on the initial Fe2+ concentration. The specific Fe2+ oxidation rates increased up to around 4 g/L and started to decrease at above 4 g/L. This implies substrate inhibition of Fe2+ oxidation at higher concentrations. Haldane equation fitted the experimental data reasonably well (R-2 = 0.90). The maximum specific oxidation rate (q(m)) was 2.4 mg/mg VS (.) h, h, and the values of the half saturation (K,) and self inhibition constants (K-i) were 413 and 8,650 mg/L, respectively. Fe2+ oxidation was competitively inhibited by Fe3+ and the competitive inhibition constant (K-ii) was 830 mg/L. The time required to reach threshold Fe2+ concentration was around 1 day and 2.3 days with initial Fe3+ concentration of 3 and 60 g/L, respectively. The threshold Fe2+ concentration, below which no further Fe2+ oxidation occurred, linearly increased with increasing initial Fe2+ and Fe3+ concentrations. Fe2+ oxidation proceeds by L. ferriphilum dominated culture at pH below I even in the presence of 60 g Fe3+/L. This indicates potential of using and biologically regenerating concentrated Fe3+ sulfate solutions required, for example, in indirect tank leaching of ore concentrates.