The rotating ring disk method (RRDE) is applied to investigate the pH effect on oxygen reduction reaction (ORR) on Ag(1 1 1) single crystal surface in 0.1 M KOH and 0.1 M HClO4. In 0.1 M KOH, the ORR proceeds through 4e(-) reaction pathway with a very small (0.5-2.5%) peroxide formation in the entire potential range. In 0.1 M HClO4 the onset potential for the ORR is shifted for ca. 400 mV toward the higher overpotentials compared to the 0.1 M KOH solution. At the low overpotentials, in 0.1 M HClO4 the ORR proceeds entirely as a 2e(-) process, i.e, 100% H2O2 formation. At higher overpotentials, the initial mixed a 2e(-) and 4e(-) reduction is followed by the potential region where the ORR proceeds entirely as a 4e(-) process, with H2O formation as a final product. The pH dependent shift in the onset of the ORR as well as the reaction pathway has been explained based on both: a thermodynamic analysis of pH independent rate determining step, and on the pH dependent change in availability of surface active sites and adsorption energies of molecular oxygen and reaction intermediates. (c) 2006 Elsevier Ltd. All rights reserved.