Ethyne dissolved in H2SO4 and HC1O(4) adsorbs on smooth and porous polycrystalline gold electrodes in a wide potential range between 0 and 1.00 V vs RHE. The adsorption process is accompanied by an anodic current transient when the adsorption potential is adjusted above the potential of zero charge. Electrooxidation and reduction reactions of the ethyne adsorbate were studied by means of differential electrochemical mass spectrometry (DEMS) and in-situ Fourier transform infrared spectroscopy (FTIRS). It was found that the only oxidation product of the adsorbate is CO2, which is confirmed by the potential-dependent mass signal for m/z = 44 in DEMS as well as the CO2 band at 2343 cm(-1) in FTIR spectra. The adsorption and oxidation charge densities provide evidence for adsorbed species that are more highly oxidized than the initial ethyne molecule. The mass signal for m/z = 26 recorded during the reduction of ethyne demonstrates that adsorbates are electrodesorbed as intact ethyne molecules. A simultaneously observed small contribution of m/z = 28 reveals the formation of ethyne in 9 parallel reaction pathway. C2HD and C2D2 were identified after the reduction of adsorbed C2H2 in 0.1 M DC10(4)/D2(O). These findings suggest that species with stoichiometries such as (C2H)(ad) and (C-2)(ad) were previously formed upon the adsorption of ethyne on polycrystalline Au in acid solution. The also- studied electrooxidation of CO showed a weaker interaction with Au than ethyne since no adsorbate remained on the surface after electrolyte exchange. It was found that an adlayer of adsorbed ethyne formed on the electrode surface completely inhibits the electro-oxidation of dissolved CO.