The influences of H+, Fe2+, Fe3+ and Cu2+ ion concentrations and their different combinations on the iron oxidation by an enrichment culture dominated by Leptospirillum-ferriphilum were studied in batch experiments and continuous-flow fluidized-bed reactors. The iron oxidation rate did not significantly vary in the pH range of 0.9-1.5 and was only partially inhibited at pH 0.7. Ferric and ferrous iron at 5 and 24 g L-1, respectively, had little or no effect on iron oxidation by the enrichment culture and the iron oxidation rate slightly decreased at C, higher concentrations. At a loading rate of 3 g Fe2+ L-1 h(-1), 2 g L-1 Cu2+ did not affect the iron oxidation rate, whereas at a loading rate of 10 g Fe2+ L-1 h(-1) it reduced the rate by 69%. The maximum iron oxidation rate was 10 g Fe2+ L-1 h(-1) at pH 0.9 and in the presence of 21 g L-1 Fe2+ and 2 g L-1 Cu2+. (c) 2005 Elsevier Ltd. All rights reserved.