The key parameters describing the operation mechanism of an active layer of PEFC cathodes were discussed. The macrohomogeneous and the agglomerate model were applied to different kinds of oxygen cathodes. From the simulations, it appears that gas pores must exist, even within real active layers thinner than 10 mu m. The more appropriate model is the agglomerate model. An effectiveness factor approach allowed determination of the main factors limiting the oxygen cathode performance. Actual solutions for further improvements of the catalyst utilization were derived from simulations.