Electrochemical reduction of Cot at metal electrodes yields CO, HCOO-, CH,, C,H,, and alcohols in aqueous media. Metal electrodes are roughly divided into two groups, CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and HCOO-ones (Pb, Hg, In, Sn, Cd, Tl). Foreign atom modified electrodes (the coverage is virtually unity) showed variable product selectivity between CO and HCOO-, which depends upon the combination of modifier atom and substrate electrode. Critical investigation of foreign atom modified electrodes derived a series of CO selectivity, as Au > Ag > Cu > Zn % Cd > Sn > In > Pb z Tl > Hg. The electrode potentials of CO, reduction are well correlated with the heat of fusion of metals and the potential of H, evolution. The order of CO selectivity agrees roughly with that of the electrode potential of CO, reduction, and is rationalized in terms of stabilization of intermediate species CO;-at the electrode surface. CO is produced from stably adsorbed CO;-, and HCOO-is formed from free or weakly adsorbed CO;-.