Reaction intermediates occurring during the oxidation of acetaldehyde were investigated by in situ infrared reflectance spectroscopy (SPAIRS and SNIFTIRS techniques). These measurements, showing that acetaldehyde was transformed into CO and CO2 successively, allowed us to choose a suitable potential program to oxidize electrocatalytically the reactant on binary and ternary platinum alloy electrodes with two compositions (Pt/Os and Pt/Ru/Os). IR results were useful to set a potential pulse program which allowed to adsorb dissociatively the organic compound. The analysis of the electrolysis products was performed by high performance liquid chromatography (HPLC). Acetic acid, formic acid and carbon dioxide were determined as the main oxidation products of acetaldehyde. (C) 2003 Published by Elsevier Ltd.