Serial sectioning by FIB/SEM and 3D reconstruction was used to study the microstructure-performance correlations for La(0.8)Sr(0.2)MnO(3-delta) (LSM) and 8 mol% yttria-stabilized ZrO(2) (YSZ) solid oxide fuel cell (SOFC) composite cathodes. In this study, the starting LSM and YSZ particle sizes were varied, keeping overall weight fractions of each solid phase constant. This analysis revealed a near one-to-one correlation between the specific parameters of the microstructure (such as the volumetric density of electrochemically-active triple phase boundaries) and cathode resistance (1/R(p)). This study has also quantitatively demonstrated that for coarser particle size and larger particle-size disparity, lower TPB lengths and surface area densities lead to lower performance. The implications for design of optimal SOFC composite electrode microstructure are discussed. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.025201jes]