The oxidation of ethanol in aqueous HClO4, was studied on co-electrodeposited PtRu electrodes in a temperature range of 5-40 degrees C by means of differential electrochemical mass spectrometry (DEMS). The formation of carbon dioxide and acetaldehyde were monitored by ion currents at appropriate masses. The presence of Ru promotes the surface reaction between the organic chemisorbate and the oxidant, whereas it may inhibit ethanol adsorption on the electrode. Optimum composition of the electrode for total oxidation was found to vary from Pt0.67Ru0.33 at 5 degrees C to Pt0.85Ru0.15 at higher temperatures.