Reduction of pedogenic iron oxides by the activity ofClostridium butyricumwas studied in model experiments under controlled conditions of pH, particle size (63 to 125 μm), glucose (2 percent), and anaerobic conditions. Three horizons—from a gley, pseudogley, and red clay soil, respectively—with different Feo:Fedratios were selected as test material. Except for Fe(II) formation, pH and Eh were measured at regular intervals. In all soils the total amount of free iron oxides (Fed), as well as the crystalline Fe (Fed- Feo), decreased while Fe(II) increased. The amount of amorphous Fe (Feo) remained relatively constant in those soils low in Feo:Fed(0.1 to 0.19), but diminished in the sample with a relatively high amount of amorphous Fe (Feo:Fed= 0.78). This suggests that crystalline rather than amorphous Fe forms are dissolved preferentially by acting as terminal electron acceptors. Nevertheless, the reduction of crystalline59Fe-labeled oxides (hematite and goethite) mixed with Fe-amorphous gley soil material (Feo:Fed= 0.78) clearly showed that oxalate-soluble Fe was attacked in preference to59Fe-labeled crystalline Fe.The processes of Fe(III) reduction in a microsite are discussed in relation to the mechanism of Fe(II) formation in flooded soils.