The influence of water adsorbates on the acetaldehyde oxidation reaction (AOR) was studied on 20 wt.% Pt/C, Rh/C, Pt-Rh/C and Pt-Rh-SnO2/C electrocatalysts by means of differential electrochemical mass spectrometry (DEMS) in an electrochemical flow cell. A polyol method was employed to synthesize the home-made electrocatalysts, the physical proprieties of which were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The investigation by cyclic voltammetry of the AOR was performed after potentiostatic formation of hydrogen- (H-ad) and oxygen-like (OHad, O-ad) adsorbates on the electrocatalysts surface at E-ad = 0.05 and 1 V vs. RHE respectively. For comparison, another pre-treatment was achieved at E-ad = 0.3 V vs. RHE, in the double layer region, where neither H+ nor OH- adsorption prevails. Comparing the CVs demonstrates that H-ad formation slows down the AOR initiation on Pt/C. The study on Rh/C reveals that water adsorbates are displaced by acetaldehyde species during their adsorption, owing to their stronger polarity. Acetaldehyde adsorption occurs quantitatively at potential as low as E = 0.05 V vs. RHE on Pt-based electrocatalysts. The reaction pathway leading to the formation of CO2 is promoted during the AOR on Rh-based electrocatalysts (Rh/C, Pt-Rh/C and Pt-Rh-SnO2/C). The lowest AOR onset potential was recorded on Pt-Rh/C. The present results on the AOR are discussed in the light of a previous similar study on the EOR. (C) 2015 Elsevier Ltd. All rights reserved.