Oxygen reduction in phosphate solution with the Cu(II) complex of 1,10-phenanthroline (phen) at a glassy carbon electrode was investigated by employing cyclic voltammetry, chronocoulometry and rotating disc electrode techniques. Cyclic voltammetric results indicated that the oxygen reduction was catalyzed by Cu(II)/Cu(I) complex of 1,10-phenanthroline in phosphate solution at a glassy carbon electrode. Choronocoulometric results showed the amount of the adsorption of [Cu(phen)(2)](2+) was increased with [Cu(phen)(2)](2+) concentration and was slightly increased with electrode potential from -0.20 to -0.90 V On the basis of measurements using rotating disc electrode together with polarization curves, Tafel slopes, and electron transfer numbers determination, the oxygen reduction with the Cu(II) complex at a glassy carbon electrode was proposed. The addition of a small quantity (10 muM) of the Cu(II) complex of 1,10-phenanthroline in the phosphate solution enhanced the electrocatalytic activity to four-electron transfer for O-2 reduction and inhibited the hydrogen evolution during oxygen reduction at a glassy carbon electrode. (C) 2003 Published by Elsevier B.V.