A comparative study was made on the reactivity of acetylene and ethylene in acids (0.05 M H2SO4 and 0.1 M HClO4) on polycrystalline Au electrodes. On-line mass spectrometry (DEMS) and in-situ Fourier Transform IR spectroscopy were used to identify reaction products. It was found that the interaction of the reactant with the electrode/electrolyte interface depends strongly on the electronic structure of the hydrocarbon. Acetylene forms a chemisorbed intermediate in a wide potential range above the potential of zero charge. Only CO2 was observed during oxidation of both adsorbed and dissolved acetylene. On the other hand, ethylene interacts more weakly with the Au surface and no strongly bonded adsorbate was found. Acetaldehyde, acetic acid and CO2 were detected as the main oxidation products of dissolved ethylene.