Electrochemical reduction of CO2 was studied using two series of single-crystal electrodes, Cu(S)-[n (111) x (111)] and Cu(S)-[n(110) x (100)] at a constant current density of 5 mA cm(-2) in 0.1 M KHCO3 aqueous solution. Copper single crystals were grown from 99.999% copper metal in a graphite crucible, and the crystal orientation was determined by the X-ray back reflection method. The surface treatment of the copper single crystal electrodes was studied in detail, and the reproducibility of the CO? reduction was greatly improved. The product distribution of the CO2 reduction varies greatly with the crystal orientation. CO2 reduction at the Cu(110) (= Cu(S)-[2(111) x (111)]) electrode gives a current yield of 20% of CH3COOH; the formation of CH3COOH in CO2 reduction has not been reported previously. The yield of CH4 was very low (6%) at the Cu(I 10) electrode. The formation of CH4 and CH3COOH changes significantly with the crystal orientation. A decrease of the step atom density in the Cu(S)-[n (111) x (111)] series reduces the yield of CH3COOH and enhances that of CH4. Introduction of the (100) step to the Cu(110) basal plane, leading to the Cu(S)-[n (110) x (100)] series with kink sites, diminishes the feature of the Cu(110). The Cu(2 10) ( = Cu(S)-[2(110) x (100)]), which has the highest number of dangling bonds of fee metals, gives a high yield of CH4 with a product distribution similar to that of Cu(111) which has the lowest density of dangling bonds.