By voltammetry with polycrystalline platinum rotating disc electrode in either nitrogen purged or oxygen saturated 1 x 10(-3) mol dm(-3) and 2 x 10(-3) mol dm(-3) HClO4 solutions with 0.1 mol dm(-3) KClO4 supporting electrolyte, the transition was observed from the proton diffusion limited to the oxygen diffusion limited rate of oxygen reduction reaction. The experiments confirmed roughly the calculation based on the Levich equation, that the limitation in the diffusion kinetics of oxygen reduction reaction by proton diffusion terminates for acid concentration higher than 1.69 x 10(-3) mol dm(-3). (C) 2014 Elsevier B.V. All rights reserved.