Catalytic CO oxidation on Pt(111) to CO2 was studied under atmospheric pressures of CO and O-2 and at various temperatures. Surface vibrational spectroscopy by sum frequency generation was used to probe the surface species, while the reaction rate and gas composition were simultaneously monitored by gas chromatography. Correlation between the turnover rates and the surface coverage of various CO species were utilized to identify the active CO species in the reaction. Ignition, above which the reaction becomes self-sustained, divides the reaction into two reactivity regimes. Below ignition, atop bonded CO appears as the major species on the surface, but the reaction rate is inversely proportional to the surface coverage of this species, indicating that it is not the active species but rather an inhibitor. The observed activation energy for the reaction in this regime suggests that desorption of atop CO is the rate-limiting step in the reaction. Above ignition, the atop CO becomes hardly detectable and the activation energy reduces to the one directly associated with the reaction energy barrier between adsorbed CO and O on Pt. In all cases, the reaction rate is linearly proportional to the surface coverage of CO adsorbed at non-registry sites or defect (or distorted) surface sites. They are therefore identified as the active CO species in this surface catalytic reaction.