The adsorption and mobility of oxygen adspecies on platinum (Pt) surface are crucial for the oxidation of surface-absorbed carbon monoxide (CO), which causes the deactivation of Pt catalyst in fuel cells. By employing nanoelectrode and ultramicroelectrode techniques, we have observed the surface mobility of oxygen adspecies produced by the dissociative adsorption of H2O and the surface reaction between the oxygen adspecies and the preadsorbed CO on the Pt surface. The desorption charge of oxygen adspecies on a Pt nanoelectrode has been found to be in proportion to the reciprocal of the square root of scan rate. Using this information, the apparent surface diffusion coefficient of oxygen adspecies has been determined to be (5.61 ± 0.84) × 10(-10) cm(2)/s at 25 °C. During the surface oxidation of CO, two current peaks are observed, which are attributed to CO oxidation at the Pt/electrolyte interface and the surface mobility of the oxygen adspecies on the adjacent Pt surface, respectively. These results demonstrate that the surface mobility of oxygen adspecies plays an important role in the antipoisoning and reactivation of Pt catalyst.