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A porous gas-diffusion electrode model considering the microstructure characteristics of the catalyst layer has been developed. The model is utilized to simulate the dc and ac responses of a reported high-performance phosphoric acid fuel cell (PAFC) cathodes developed by Watanabe et al. [J. Electroanal. Chem., 195, 81 (1985)]. For these electrodes with 30 and 40% poly(tetrafluoroethylene) in the catalyst layers, the dc performance predictions are in good agreement with the experimental behavior. The dc modeling and simulation results suggest that the necessary microstructure characteristics in the catalyst layer of high performance gas diffusion electrodes for PAFCs include (i) a sufficient amount of electrolyte to fill in the primary pores and to wet the secondary pore surface on the condition that no electrolyte flooding takes place in the secondary pores and (ii) a large secondary-pore surface area available in the catalyst layer. The qualitative characteristics of the catalyst layers of these electrodes are analyzed and interpreted by means of the simulated electrode ac responses at several electrode operating conditions.