(La 0.8Sr 0.2)MnO 3-Y-stabilized zirconia (LSM-YSZ) cathodes fired at various temperatures were studied using 3D Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) tomography and Impedance Spectroscopy (IS). The lowest resistance cathode, fired at 1175°C, had nearly the highest active three-phase boundary density. This optimum was where the firing temperature was sufficient to yield good LSM-particle percolation but not so high as to cause substantial particle coarsening and densification. Increasing the firing temperature above 1175°C caused a shift in the main impedance response from ∼100-1000 Hz to 1-10 Hz; this low-frequency response was primarily an electrochemical process, with gas diffusion becoming significant only for low oxygen pressure. Focusing on the low frequency peak for cathodes fired at 1325°C, A-site-deficient LSM yielded lower resistance than stoichiometric LSM. Finally, cathodes were annealed at 1000-1100°C in order to accelerate structural evolution; changes in the impedance response were similar to those observed with increasing firing temperature.