Two forms of the methanol electro-oxidation intermediate with stoichiometry C:H:O, COH (hydroxymethylidyne) and HCO (formyl), on Pt (111) with and without coadsorbed water were studied using density functional theory calculations. The structure, adsorption energy and stability with respect to dissociation were calculated. Both HC=O and C-OH were stable on clean Pt (111) and with a single coadsorbed water molecule, while only the HCO configuration was stable in the presence of a whole water layer. The vibrational modes of HC=O on a bridge site showed no mode around 1700 cm (1) characteristic of C=O stretch making it hard to distinguish it from C-OH. (C) 2012 Elsevier B. V. All rights reserved.