Porous electrodes provide high-surface-area supports for the catalysts of many reactions, but the influences of electrode preparation conditions on electrocatalysts are not always well understood. Electrochemical impedance spectroscopy (EIS) can provide extensive information about an electrode, but the models describing the spectra are often too idealized to draw useful conclusions. We describe a new model based on an array of parallel, nonuniform transmission lines for predicting the response of porous electrodes. The model incorporates physically realistic elements, such as discrete particles of variable size and adjustable multilayer stacking geometries. Resistance parameters were derived from experimental data for Pt4Ru4Ir-coated Ti0.9Nb0.1O2 and Ebonex electrodes prepared under varying degrees of oxidative conditioning. The results, which indicate a high degree of impedance at the support-solution interface and consequently, low catalyst utilization, suggest several strategies for improved electrode design. (C) 2003 The Electrochemical Society.