The effects of adsorbed sulfide ion (S2-) formed on metal electrodes in the electrochemical reduction of CO2 under high pressure were investigated. The selectivity for the reduction products changed markedly with the Na2S treatment of metal electrodes. In the case of a Cu electrode, the faradaic efficiency for methane formation decreased remarkably from 36% to 2%, whereas that for C2 components (ethylene and ethanol) increased from 13% to 25% due to the Na2S treatment. The faradaic efficiencies for formic acid and hydrogen formation also increased with the treatment. A similar change in the product selectivity was also observed in the electrolysis with addition of Na2S into the aqueous electrolyte. A study of the current-potential curve suggested that adsorbed sulfide ion prevented the formation of adsorbed CO and thus changed the product selectivity.