High-resolution x-ray photoelectron spectroscopy has been used to study the kinetics of the CO oxidation reaction on a Pt(111) surface in situ. The study focuses on the interaction of a preadsorbed p(2x2) layer of atomic oxygen with CO dosed using a supersonic molecular beam. Measurements of O 1s and C 1s spectra at 120 K show that CO adsorbs on the oxygen precovered substrate, but no reaction occurs. A maximum CO coverage of 0.23 ML (monolayer) is observed, with CO exclusively bound on on-top sites. In accordance with the literature, bridge sites are blocked by the presence of atomic oxygen. The reaction of CO with preadsorbed O to CO2 is studied isothermally in a temperature range between 275 and 305 K. The reaction rate initially increases with CO pressure, but saturates at 9x10(-7) mbar. The data indicate that a certain amount of disordered oxygen within the p(2x2) layer acts as a starting point of the reaction and for a given temperature reacts with a higher rate than O in the well-ordered oxygen p(2x2) phase. For the reaction of CO with this ordered phase, the results confirm the assumption of a reaction mechanism, which is restricted to the edges of compact oxygen islands. The activation energy of the reaction is determined to (0.53+/-0.04) eV, with a prefactor of 4.7x10(6+/-0.7) s(-1). (C) 2004 American Institute of Physics.