pH effect on oxygen reduction reaction (ORR) at Pt(1 1 1) electrode has been investigated in 0.1 M HClO4, 0.1 M NaOH, 0.5 M NaClO4 + x mM HClO4 (x = 0.1-35 mM) and 0.5 M NaClO4 + y mM NaOH (y = 2-20 mM) solutions using hanging meniscus rotating disk electrode system (HMRDE). We found that (i) during ORR in solutions with 2.5 < pH < 4 and low buffer capability, when the mass transport for H3O+ is not fast enough to maintain O-2 + 4H(+) + 4e(-) -> 2H(2)O (reaction (1)), O-2 + 2H(2)O + 4e(-) -> 4OH(-) (reaction (2)) will set in, which is accompanied with sensitive change of the pH at electrode/electrolyte interface (pH(s)); (ii) at the same overpotential, ORR current density (J(ORR)) is approximately the same for both reactions (1) and (2) in all solutions irrespective of pHs; (iii) the kinetic and kinetic-mass transport mixed-control regions for ORR locate in the same potential region for OHad + H+ + e(-) reversible arrow H2O or OHad + e(-) reversible arrow OH- in O-2 free solution, both ORR and OHad adsorption/desorption display similar pH independence; (iv) During ORR, OHad adsorption/desorption occurs fast in both the forward and backward direction in parallel with ORR. The results reveal that the high activation overpotential and the slow kinetics for ORR at Pt(1 1 1) are limited by the thermo-electrochemistry of OHad adsorption/desorption. (C) 2013 Elsevier Ltd. All rights reserved.