Abstract <p>Using elemental analysis, X-ray diffraction, and high-resolution transmission electron microscopy, it was shown that nanocrystals of M–Pt solid solutions with a face-centered cubic structure are preferably formed during the coreduction of metal precursors (<i>M</i> ²⁺ (<i>M</i> = Fe, Co, Ni) and [PtCl₆]^2–) by an alkaline hydrazine hydrate solution with the predominant (during synthesis) content of Fe and Co in the phase of solid solutions being ≈11.5 ± 0.5 and ≈16.9 ± 1 at %, respectively. A comparison of the results of high-resolution transmission electron microscopy, elemental analysis, X-ray phase analysis, and X-ray structural analysis demonstrated that in the Fe–Pt and Co–Pt systems, in addition to the M–Pt solid solutions with the face-centered cubic structure, which were revealed by X-ray diffraction methods, nanodispersed metal phases, practically inaccessible for registration, were formed in regions above and below the limits of Fe and Co contents. However, in the nanostructured Ni–Pt system, there was no upper limit of Ni content in the solid solutions of the face-centered cubic structure up to 40 at %. Therefore, the phase compositions were represented by two types of face-centered cubic structures: a pure Ni phase and solid solution phases with a Ni content of 10–12 and 40 at %. The ideas about the nature of these structural-phase features are presented.</p>