Potential dependence of adsorbed water, CO species, formate and perchlorate ions on polycrystalline Pt electrode chemically deposited on a Si substrate has been investigated by in situ ATR-SEIRAS (surface-enhanced infrared absorption spectroscopy) in 0.1 M HClO4 in the presence of 0.1 M methanol. The newly appeared adsorbed water with O-H stretching vibration around 3658 cm(-1) co-exists with adsorbed CO derived from methanol and its consumption during oxidation of the adsorbed CO and increase of formate species above ca. 0.6 V is demonstrated in a quantitative way by a linear relationship between the intensity of the adsorbed water and those of linear CO and formate. This can be interpreted as a strong proof that the water molecules which appeared upon methanol adsorption and co-exist with the resultant CO is in fact the species which react with the CO and promotes further methanol oxidation. (c) 2005 Elsevier B.V. All rights reserved.