Using the technique of Cs+ reactive ion scattering (Cs+ RIS), we show that reaction of CO and O on a Pt(111) surface produces a CO2 precursor species that remains stable on the surface up to 300 K. The CO2 species desorbs with an activation energy of 46+/-3 kJ mol(-1), whereas the direct CO2 gas formation via Langmuir-Hinshelwood (LH) reaction between CO and O occurs with an activation energy of 75+/-4 kJ mol(-1). The observation reveals a new pathway for CO oxidation reaction, which is different from the direct LH pathway.