Oxygen incorporation for the oxygen reduction reaction at a porous mixed-conducting LaSrCoO3 (LSC) electrode was experimentally investigated using technique of oxygen isotope (O-18) labelling coupled with subsequent quenching and secondary ion mass spectrometer (SIMS) imaging. The isotopic oxygen exchange experiments of porous LSC cathode were conducted under current density conditions of 0 (OCV), 0.05 and 0.24 A cm(-2). Distribution images of incorporated O-18 inside solid oxide microstructures and analyzed concentration profiles along the LSC cathode thickness are presented. The O-18 distribution image and concentration profile under an applied current density clearly reflect extension of active reaction zone over surface area of LSC, as well as distribution of oxygen chemical potential in the porous LSC cathode. The non-uniform profiles under influence of applied currents revealed that oxygen chemical potential distribution in porous mixed-conducting LSC cathode strongly depends on an applied current density. The analyzed profiles also showed a significant interfacial barrier to the oxygen diffusion across the interface between the GDC interlayer and the YSZ electrolyte due to the unfavorable diffusion of Sr from the LSC to the GDC/YSZ interface. The present results provide the first experimental evidence of oxygen chemical potential distribution in the porous mixed-conducting LSC cathode. (c) 2019 The Electrochemical Society.