The polarization resistance of composite electrodes for solid oxide fuel cells was modeled by three-dimensional random-resistor networks. These were generated on a computer by identifying neighbors in cubic lattices randomly occupied by electrolyte particles (ionic conductors) or electrode particles (electronic conductors), or in random packings generated by sequential deposition of such particles in random order. The polarization resistances between electrode and electrolyte particles were taken to be in parallel with interfacial capacitances, and the polarization resistance of the composite was calculated as the difference between high- and low-frequency resistance of the resistor networks. The volume fraction of electrode particles at which the minimum in polarization resistance occurs was found to increase with the ratio between electrode-particle radius and electrolyte-particle radius. This was rationalized by investigating the limits within which the composite may be expected to contain electrode-electrolyte interfaces in which both the participating clusters extend throughout the composite. if such interfaces are present, there will be a thickness dependence in the polarization resistance to a degree depending on the component conductivities and polarization conductances, otherwise not. The results are in reasonable agreement with experimental data.