A comparative study on the Fe-reducing ability of pure anaerobic strains of Geobacter metallireducens and Bacillus infernus was investigated using different sources of Fe(III). Batch tests were carried out in aqueous solutions containing a reducing agent (lactate or formate) and at a constant temperature of 37°C and 50°C respectively. The formation of biogenic compounds of Fe(II) was determined using XRD and SEM-EDX techniques. The bioreduction of magnetite was not affected by the type of bacterial strain used. The kinetics of the process, initially very fast, stopped rapidly for both types of microorganisms. Vivianite (Fe3(PO4)2.8H2O) was detected as the main biogenic compound formed during the bioreduction process.