(La0.8Sr0.2) MnO3-Yttria Stabilized Zirconia (LSM-YSZ) cathodes fired at various temperatures were studied using Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) three-dimensional (3D) tomography and Electrochemical Impedance Spectroscopy (EIS). The total cathode polarization resistance, measured at 800 degrees C in air, showed a minimum versus firing temperature, T-f, at 1175 degrees C. The EIS showed two dominant responses that were fit well using a two (R-CPE) element equivalent circuit. The higher frequency (10(4)-10(5) Hz) response, attributed to YSZ grain boundary resistance within the LSM-YSZ composite, decreased with increasing T-f and was explained by grain size increases estimated from the 3D structural data. The main EIS response, attributed to the oxygen reduction process, decreased in characteristic frequency from 500 to 1 Hz as T-f increased, while its magnitude was minimized at 1175 degrees C. An electrochemical model quantitatively predicted the resistance minimum based primarily on a maximum in the density of electrochemically-active three-phase boundaries (TPBs), measured using 3D tomography. The active TPB density maximum resulted from two factors: substantial particle coarsening and densification at high T-f that yielded a low TPB density, and low LSM-particle percolation at low T-f that yielded a low fraction of active TPBs. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.053204jes] All rights reserved.