Glucose oxidation in a 0.1 M NaOH solution at Au(111) single crystal electrodes modified with various ad-metals (Cu, Ag, Ru, Pt, Pd and Cd) by underpotential deposition (upd) was investigated. Ag ad-atoms of 1/3 monolayer (ML) onto the Au(111) surface showed the highest catalytic activity for glucose oxidation: The peak potential for glucose oxidation to gluconolactone (two-electron oxidation) was around -0.40 V vs. Ag\AgCl\KCl(sat), and the catalytic effect of this electrode on the oxidation of gluconolactone was also observed around -0.1 V vs. Ag\AgCl\KCl(sat) to give formate as the main product in a 0.1 M NaOH solution. Full coverage of Ag ad-atoms, however, led to a decrease in peak current with no marked shift in potential for glucose oxidation, but suppressed the oxidation of gluconolactone significantly. Cu ad-atoms modified Au(111) electrodes showed a slight negative shift of the pre-peak in the oxidation of glucose with a small oxidation current and a main oxidation peak appeared around -0.04 V vs. Ag\AgCl\KCl(sat). Cd ad-atoms gave similar activity at a bare Au(111) electrode, whereas Ru and Pt ad-atoms showed a considerable decrease in oxidation current. Pd ad-atoms gave negative catalytic activity. Double layer capacitance measurements revealed that the shift of the potential of zero charge (pzc) between bare Au(111) and Ag ad-atom modified Au(111) electrodes (the pzc shifted negatively by modification of Ag) was of great help in the explanation of the observed catalytic activities, where the number of AuOH sites on the Au(111) surface would be important for the oxidation of glucose and gluconolactone. (C) 2004 Published by Elsevier B.V.