Dense microelectrodes fabricated with well-defined microstructures and geometries using microelectronic fabrication techniques were studied using electrochemical impedance spectroscopy (EIS) to investigate the rate-limiting reaction step(s) for oxygen reduction reaction (ORR) on the La0.8Sr0.2MnO3 (LSM)/8 mol % Y2O3-stabilized ZrO2 (8YSZ) system. At least four distinct reaction processes were observed for ORR on LSM/8YSZ under ambient PO2 and at intermediate temperatures, which have been assigned tentatively to: (i) ion transport in 8YSZ with an average activation energy of 1.16 +/- 0.02 eV, (ii) a surface diffusion process on LSM with activation energy in the range from 1.34 to 1.65 +/- 0.03 eV, (iii) at least one surface chemical process on LSM with activation energies in the range from 1.71 to 1.88 +/- 0.02 eV and an average capacitance value of 3.4x10(-4) F/cm(2), and (iv) a mixed bulk/three-phase boundary (TPB) charge transfer process with activation energies in the range from 2.42 +/- 0.02 to 3.05 +/- 0.03 eV and an average capacitance value of 3.2x10(-3) F/cm(2). This study suggests that the overall ORR rate may be limited by mixed bulk/TPB charge transfer processes below 700 degrees C and surface chemical reactions above 700 degrees C. The ORR current contributions from the bulk oxide ion transport and TPB incorporation path was estimated by correlating microelectrode geometry to impedance data. (c) 2007 The Electrochemical Society.