Catalysts containing heterostructures consisting of composite metal/metal oxides have been proven to be very active for many catalytic reactions. In this work, Pt/FeOx heterostructures, including Pt/Fe2O3 and Pt/Fe3O4 nanospheres, were synthesized and used for catalytic oxidation of CO. Fe2O3 and Fe3O4 nanospheres were synthesized by hydrothermal methods, and then platinum precursors were in situ reduced on the prepared FeOx in ethylene glycol solutions. The results of the activity test showed that the Pt/Fe3O4 nanospheres exhibited much higher catalytic activity of CO oxidation than the Pt/Fe2O3 nanospheres over the entire temperature range of 50 degrees C-225 degrees C. Complete CO oxidation could be achieved on the Pt/Fe3O4 catalysts at temperatures above 150 degrees C, while the full CO conversion required a temperature of 225 degrees C on the Pt/Fe2O3 catalysts. Several characterization techniques were used to investigate the physicochemical properties in light of the activity differences of the prepared catalysts. The combined results of TEM and XRD showed that Pt/Fe3O4 and Pt/Fe2O3 were comprised of Pt nanoparticles with an average crystallite size of 3.7 nm and 3.6 nm, respectively. The Pt nanoparticles could uniformly distribute throughout the FeOx nanosphere supports. According to XPS and H-2-TPR characterizations, surface chemisorbed oxygen was readily formed on the Pt/Fe3O4 nanospheres, and a synergistic interaction might exist between Pt and Fe3O4 nanospheres. The presence of Pt nanoparticles facilitated the activation of surface chemisorbed oxygen, thus mainly contributed to the CO oxidation. In situ DRIFTS results indicated that the oxygen activation was a rate-limiting step in the catalytic oxidation process. Linearly adsorbed CO species on the Pt sites were then oxidized into CO2.