Properties of Pt0.5M0.5 nanopowders (M = Fe, Co, Ni) of alloys obtained via the decomposition of double complex salts [Pt(NH3)5Cl][Fe(C2O4)3] ∙ 4H2O, [Pt(NH3)4][Co(C2O4)2(H2O)2] ∙ 2H2O, and [Pt(NH3)4][Ni(C2O4)2(H2O)2] ∙ 2H2O, respectively, are studied in the reaction of preferential CO oxidation. It is shown that bimetallic Pt0.5M0.5 catalysts (M = Fe, Co, Ni) are much more active in the low temperature range than Pt nanopowder. The activity of the catalysts decreases in the order Pt0.5M0.5 ≥ Pt0.5M0.5 > Pt0.5M0.5 @ Pt. The higher activity of bimetallic Pt0.5M0.5 catalysts in the reaction of preferential CO oxidation in the low-temperature range under conditions of dense Pt surface coverage by adsorbed CO molecules is most likely caused by the activation of CO on Pt atoms, the activation of O2 on atoms of the second metal (Fe, Co, Ni), and the reaction that occurs at the sites of contact between the atoms of platinum and the atoms of the second metal on the surfaces of the alloy’s nanoparticles. The bimetallic systems investigated in this work can be used to improve catalysts of practically important preferential CO oxidation reaction. These systems have considerable potential in the afterburning reactions of CO and hydrocarbons; hydrogenation reactions; electrochemical reactions; and many others. The means used in the preparation of bimetallic nanopowders based on the decomposition of double complex salts is simple, does not require the use of expensive or complex reagents, and can be easily adapted to produce supported catalysts containing Pt0.5M0.5 metal alloys (M = Fe, Co, Ni).