Methanol adsorption/oxidation on a polycrystalline platinum electrode was investigated by ATR-FTIRAS combined with linear potential sweep voltammetry after initial adsorption at 0.05 V in 0.05-1.0 M HClO(4) containing 0.04-1.0 M methanol. Methanol adsorption exhibits a pronounced maximum around 0.56 V, which coincides with the onset potential of the oxidation. This suggests that adsorbed CO is not a catalytic poison but an intermediate of the oxidation. The low methanol oxidation activity on Pt is not ascribed to the poisoning action by the adsorbed CO but to inability of Pt to dissociate water to adsorbed OH species needed for the CO and methanol oxidation. It was found for the first time that methanol adsorption at 0.05 V is significantly diminished in dilute perchloric acids. This was interpreted in terms of reduced physisorption of methanol at the adsorption potential which becomes more negative with respect to the pzc. Band intensity of the formate species with a symmetric O-C-O vibration around 1320 cm(-1) remains unchanged with increase of methanol concentration from 0.04 M to 1 M, while the peak current of methanol oxidation increases by ca. three times in 1 M HClO(4). The result suggests a minor role of the formate in the methanol oxidation mechanism. (C) 2009 Elsevier B.V. All rights reserved.