The oxidation of methanol by molecular oxygen on a Pt(111) surface has been investigated by infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy. Formate is produced when the surface co-adsorbed with molecular oxygen and methanol is annealed to 70 K; the temperature is far much lower than the dissociation temperature of molecular oxygen on a clean Pt(111) surface. The attractive interaction between the co-adsorbates is postulated to lower a dissociation barrier of molecular oxygen. When a methanol-precovered Pt(111) surface is exposed to O-2, the sticking probability of O-2 decreases with increase of surface temperature and the major product of methanol oxidation is changed from formate to CO. (C) 2004 Elsevier B.V. All rights reserved.