Electrochemical oxidation of carbohydrates at several copper-oxide-modified electrodes in alkaline solution has been studied using cyclic voltammetry and rotating-disk voltammetry. It has been found that Cu(II) oxide on the electrode surface is essential for the electrode process, in which some electrogenerated higher Cu(III) oxide seems to serve substantially as a catalyst. Each mole of xylose, glucose and sorbitol was found to be oxidized to 5 mol, 6 mol and 6 mol of formic acid respectively with the transfer of 10, 12 and 14 electrons. The Cu(II)-oxide-modified electrodes allowed the highly sensitive constant-potential amperometric detection of carbohydrates down to picomole level in flow-through systems.