The thermal treatment of chains composed of amorphous Fe 1–x Co x nanoparticles in two different oxygen atmospheres was studied. The nanostructures were manufactured using a magnetic-field-induced coreduction reaction, in which the precursor solutions containing 1:3 and 3:1 proportions of Fe 2+ and Co 2+ ions were reduced with sodium borohydride. The as-prepared nanochains were then heated for 30 min at 400 and 500 °C in dry air or argon containing about 1% oxygen. These processes led to their oxidation, and, as a result, the thermally treated Fe 1–x Co x nanochains were transformed into cobalt ferrite. Heating at 500 °C in the air-containing atmosphere caused the nanomaterials to lose their nanochain structures. In accordance to room-temperature magnetic measurements, the as-prepared and thermally treated Fe 1–x Co x nanochains were ferromagnetic. The highest saturation magnetization ( M S ) was measured for the Fe 1–x Co x nanochains treated at 400 °C in dry air (105 A·m 2 /kg and 154 A·m 2 /kg for Fe 0.25 Co 0.75 and Fe 0.75 Co 0.25 , respectively), whereas the lowest M S was found for the Fe 0.25 Co 0.75 heated at 500 °C in dry air (17 A·m 2 /kg) and the Fe 0.75 Co 0.25 heated at 500 °C in argon (16 A·m 2 /kg).