Аннотация:X-ray diffraction, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and magnetic studies of Fe50Co50, Fe65Co35 and Fe35Co65 alloys have been performed after different isothermal heat treatments of the alloys at 500–1200°C. It is shown that in the A2 region of the diagram the separation into fcc Co and bcc Fe occurs in a thin surface layer, and partially in the bulk (5–10%). The conclusion is made that B2 ordering of the alloy quenched starts with the precipitation of globular particles consisting of the species, which are in excess relative to the stoichiometric composition of the B2 phase. Prior to the B2 ordering onset, the separation-ordering phase transition occurs; this is accompanied by changes in the electronic structure of the valence band of the Fe50Co50 alloy. The so-called “550°C anomaly” is the result of the phase transformation, which proceeds according to a martensitic mechanism and leads to a small increase of the lattice parameter. The conclusion is made that the dependence of the magnetic properties of Fe-Co alloys on B2 ordering is not clearly expressed.