A micromodel based upon percolation theory is developed to predict effective properties in composite electrodes for solid oxide fuel-cell (SOFC) applications. The theory considers binary and multi-component mixtures of particles that are either ion or electron conductors. The model predicts effective ionic and electronic conductivities, three-phase boundary lengths, and hydraulic pore radii. The effective properties depend upon primary physical characteristics, including average particle-radii, volumetric packing densities, particle contact angles, and porosity. All results ate presented in nondimensional form, which provides considerable generality in their practical application. (C) 2009 Elsevier B.V. All rights reserved.