The formation of Fe-Co nanoparticles and nanowires via electroless deposition under a magnetic field was electrochemically investigated by an in-situ mixed potential measurements, quartz crystal microbalance measurement, and cyclic voltammetry. These electrochemical measurements revealed that the formation of Fe-Co nanoparticles in the metallic salt solution is due to the disproportionation reaction of Fe(II). Addition of hydrazine solution results in the nucleation and growth of Fe-Co particles. The deposition rate becomes lower at a higher concentration of Fe(II) owing to the lower catalytic activity of iron for the oxidation reaction of hydrazine, which affects deposition behaviors such as morphology and yield. In the presence of a magnetic field, Fe-Co nanowires can be obtained by hydrazine reduction. The formation of Fe-Co nanowires is strongly affected by the deposition rate; pure cobalt and Fe-Co alloy nanowires with smooth surfaces are formed because of their relatively high deposition rate, while iron wires are difficult to fabricate by hydrazine reduction because of their quite low deposition rate. Based on the analysis of the nanowire formation processes in the Fe-Co system, iron nanowires were successfully fabricated by a disproportionation reaction under a magnetic field. (C) 2013 The Electrochemical Society.