Combined isotope-exchange and electrochemical experiments have been carried out to study the oxygen reduction reaction of a solid oxide fuel cell cathode under different operation conditions. For a test cell, LaNi0.6Fe0.4O3 (LNF) is chosen as a porous cathode material which was sintered on an Y2O3-ZrO2 electrolyte with a Ce0.9Gd0.1O1.95 interlayer. Comparisons are made in the isotope oxygen diffusion profiles between an OCV and a polarized (J = 0.07 A/cm(2)) conditions by taking into account the oxygen diffusion and surface exchange properties of bulk LNF as well as accumulated results on oxygen isotope profiles. Considerations have been made to account for the concentration up-hill diffusion in the GDC interlayer under the OCV conditions and other characteristic features, leading to recognition that the oxygen reduction reaction in the perovskite cathode and GDC interlayer exhibits complicated behaviors caused probably by several different mechanisms. (C) 2014 Elsevier B.V. All rights reserved.