The Pt0.5Со0.5/SiO2 catalyst has been prepared by the decomposition of a [Pt(NH3)4][Co(C2O4)2(H2O)2]. 2H2O binary complex salt supported in the pores of SiO2 pellets. It has been shown by a complex of physical and chemical methods that Pt0.5Со0.5/SiO2 contains alloy nanoparticles with an average composition Pt0.5Co0.5. The catalytic properties of Pt0.5Со0.5/SiO2 are studied in the preferential oxidation of СО in the reaction mixtures with various compositions. It was found that Pt0.5Со0.5/SiO2 has a high selectivity and makes it possible to decrease the outlet concentration of CO to a level of <10 ppm, and the presence of СО2 and/or Н2О in the reaction mixture almost does not affect its catalytic properties. The structure of the catalyst is stable under the conditions of preferential CO oxidation.