The electro-oxidation and electroreduction of ethene on polycrystalline Au were studied by means of differential electrochemical mass spectrometry (DEMS). It was found that the electroreduction of ethene in 0.1 M HClO4 + H2O yielded ethane whereas in DClO4 + D2O deuterated ethane C2H4D2 was detetected. The electro-oxidation of C2H4 was accompanied by a large signal for m/z = 44 in the mass spectrometer owing to the formation of both CO2 and acetaldehyde. Using C2D4 in H2O + HClO4 the production of CO2 and ethanal was distinguished. Experiments with C2H4 in DClO4 + D2O demonstrated that all hydrogen atoms in ethanal belong to the initial C2H4 molecule, thus indicating an internal rearrangement during ethene electro-oxidation.