Electrocatalytic ethyne oxidation and reduction on Pt was studied by on-line mass spectrometry (DEMS). It was shown that the initial step in both processes involves a rapid reactant adsorption on the metal surface. The adsorbate consists of molecular ethyne, partially dehydrogenated hydrocarbon residues C2Hx and C-2-oxygen-containing species in a ratio dependent on the electrode potential. The maximum electrode coverage is achieved in the potential range 0.2-0.4 V, whereas the major part of the adsorbate is ethyne itself. Ethane was identified as the main volatile product of the cathodic hydrogenation of the surface species. A trace amount of 1-butene was formed in a slow side reaction. Evolution of a mixture of ethane and ethene was observed in the presence of bulk reactant. Measurements at various sweep rates revealed that electrooxidation of the preadsorbed ethyne to CO2 at E > 0.8 V is much faster than its electroreduction in the hydrogen potential region. The optimal conditions for monitoring and/or utilization of ethyne in an electrochemical cell working in a programmed step potential mode are specified.