CeO₂ has been extensively studied as a support material for platinum group metals because of the high catalytic activity of CeO₂-supported catalysts in oxidation reactions. In particular, it has been reported that the Pt/CeO₂ catalyst exhibits high CO oxidation activity after reductive treatment, thereby lowering the temperature of 50 % CO conversion to 150 ℃ or less. Such high activity of Pt/CeO₂ has been attributed to the formation of active sites at the interface between the Pt nanoparticles (NP) and CeO₂ surface. In this study, we attempt to deepen our understanding of the catalytic activity of Pt/CeO₂ by studying the spillover of CO and O species at the interface. Combined kinetics and in situ CO-diffuse reflectance infrared Fourier transform spectroscopy studies reveal that CO spills over from Pt NPs to the CeO₂ surface; this contributes to circumventing Pt poisoning by CO at low temperatures. Moreover, the CO oxidation activity of the Pt/CeO₂ catalyst was considerably enhanced by simply mixing CeO₂ particles with the Pt/CeO₂ catalyst. This is explained by the formation of more reactive O atoms on the CeO₂ surface after the reductive treatment. This study demonstrates that understanding the dynamic mobility of reactants is vital for discerning catalyst activity and designing more reactive catalysts.