A series of CoO_x–FeO_x composite oxides with different Co content was prepared using the hydrothermal method. Low-temperature activity of CoO_x–FeO_x composite oxides for CO oxidation was significantly improved by the addition of 50 atm % CoO_x into FeO_x. Further increase in Co content up to 70 atm % caused a decrease in low-temperature activity. Structural characterizations by X-ray diffraction, transmission electron microscope and Fourier transform infrared (FT-IR) revealed that Co species, which were not substituted with Fe sites to form CoFe₂O₄, are highly dispersed on the surface of CoFe₂O₄ particles with a perimeter interface for the samples with 50 atm % CoO_x. The surface valence state of CoO_x–FeO_x composite oxides was found to be different by FT-IR spectroscopy following NO adsorption. CoO_x–FeO_x with 50 atm % CoO_x includes a relatively large amount of quasi-tetrahedrally coordinated Co²⁺ sites on the surface, whereas that with 10 atm % CoO_x consists of Fe²⁺ sites on the surface. Temperature-programmed reduction by H₂ measurements suggested that CoO_x–FeO_x with 50 atm % CoO_x possesses the largest amount of active oxygen species, which can be reduced by H₂ in the lower temperature region. The surface oxygen species of which the formation is related to the presence of quasi-tetrahedrally coordinated Co²⁺ sites was concluded to participate in CO oxidation reaction on CoO_x–FeO_x composite oxides.