The electrocatalytic reduction of molecular oxygen (O-2) has been performed in O-2-saturated 0.5 M KOH solution at Au nanoparticles electrodeposited onto two different carbon substrates, namely glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG). Cyclic voltammetry (CV) technique has been used in this investigation. The electrocatalytic activity of the An nanoparticle-based electrodes is inherently related to its electrodeposition conditions (i.e., the absence or presence of some additives) as well as the nature of the substrate. For instance, Au nanoparticles electrodeposited onto GC (nano-Au/GC) from K[AuBr4] in the presence of 25 mu M cysteine showed a high electrocatalytic activity towards the oxygen reduction reaction (ORR) as demonstrated by the largest positive shift of the cathodic peak potential (at ca. -0.165 V versus Ag/AgCl/KCl (sat)). On the other hand, two well-separated successive reduction peaks corresponding to the 2-step 4-electron reduction of oxygen were observed at the different nano-Au/HOPG electrodes. The relative ratio of the two peak current heights changed significantly depending on the electrodeposition conditions of the An nanoparticles. The morphology of the different Au nanoparticles electrodeposited onto the different substrates was depicted by scanning electron microscope (SEM) technique. (c) 2006 Elsevier Ltd. All rights reserved.