Direct biotic and homogeneous abiotic Fe(II) oxidation rates as well as oxidation rates of Fe(II) with MnOX were determined in laboratory experiments and compared with biotic Mn(II) oxidation rates. In groundwaters and thermal installation waters, parameters for both Fe(II) oxidation steps were studied and products of biotic and abiotic Fe(II) and Mn(II) oxidation were analyzed. Direct Fe(II) oxidation of active Leptothrix cholodnii cultures can reach first-order rates of up to 1.17 ± 0.90 h−1. Second-order rates of Fe(II) oxidation with biogenic, Leptothrix-cholodnii- and Leptothrix-discophora-SS-1-originating MnOX lead to rates comparable with those as obtained with abiotic H+-birnessite.