The electrochemical behavior of ethanol and acetaldehyde was studied at rhodium electrodes in acid solutions using electrochemical mass spectrometry (DEMS). Acetaldehyde and CO2 were detected from bulk ethanol, whereas only CO2 was recorded from acetaldehyde. The production of acetic acid could not be confirmed with this method. The formation of methane was established from both molecules, which suggests that the fragmentation of the C-C chain occurs during the electroreduction and/or adsorption processes. Adsorbed species were isolated by applying a flow cell procedure. The residues produce CO2 and methane during oxidation and reduction reactions, respectively, the amount of these compounds depending on the adsorption potential. By use of isotopic labeled (CH3CH2OH)-C-12-C-13 and (CH3CHO)-C-12-C-13, the contribution of each C atom in the adsorbate was distinguished. It was established that the CH3 group oxidizes at potentials within the 0 adsorption potential region of rhodium during the positive-going sweep and also at E < 0.60 V during the reverse scan, The CH2OH or CHO groups yield CO2 in the same potential region as COad. The influence of the anion on these reactions was established using sulfuric and perchloric acids as base electrolytes, being noticeable for ethanol adsorption. Compared to Pt, Rh electrodes exhibit an enhanced activity for deprotonation and C-C cleavage reactions.