The oxidation of methanol on a Pt(111)-(2x2)O surface has been investigated by infrared reflection absorption spectroscopy and temperature-programed desorption. Methanol is dehydrogenated to produce methoxy species in the annealing temperature range from 130 to 170 K. Above 170 K, the reaction proceeds differently, depending on methanol coverage. At the saturation coverage, methanol adsorbates partly desorb molecularly and partly react with precovered oxygen atoms to produce CO, H-2, and H2O. No detectable formaldehyde or formate is formed. In contrast, at submonolayer coverages, methoxy species is dehydrogenated to yield formaldehyde at similar to180 K and further oxidized to formate at similar to200 K. Formate is decomposed by 300 K. Defect sites such as steps are not relevant to the formation of the intermediates. When CO is coadsorbed on the surface, it destabilizes the reaction intermediates. The destabilization by coadsorbed CO makes the reaction intermediates short lived so as not to be detectable at high initial coverages of methanol. (C) 2003 American Institute of Physics.