We propose a method for the characterization of poisoning on the surface of a Pd electrode during hydrogen evolution using the reaction between deuterium emerging from the electrode bulk and hydrogen adsorbed by electrolysis of water. The method was applied to the poisoning by carbonaceous adspecies forming during CO2 electrochemical reduction in 0.05 M Li2CO3, 0.1 M NaHCO3 and 0.1 M KHCO3 solutions. In comparison with N-2 atmosphere, the exchange reaction was suppressed in CO2 electrochemical reduction for all electrolytes. The degree of the suppression followed the order Na+> Li+> K+. Deuterium emergence from the subsurface was enhanced only in KHCO3, in which formic acid production was observed. The electrolyte in which CO formed most was NaHCO3 and, in this case, deuterium from the subsurface was largely suppressed. The relation between the surface hydrogen processes and the selectivity of the electrochemical reduction of CO2 is discussed.