Electrocatalytic oxidation of formaldehyde on Cu single crystal planes was investigated by cyclic voltammetry in aqueous 0.1 M NaOH solutions. The catalytic activity of the Cu(110) surface appeared to be significantly higher than that of Cu(100) and Cu(lll) planes. The surface roughness and the dislocation density of Cu single crystal electrodes were shown not to be responsible for a different catalytic activity of a Cu single crystal plane in the formaldehyde oxidation reaction. A more pronounced surface imperfection facilitates a surface passivation process. A different electrocatalytic activity of Cu single crystal planes may be accounted for by a different geometrical arrangement of surface atoms.