The kinetics of oxygen reduction reaction on platinum/carbon powders in a Nafion film were evaluated with rotating disk electrode and gas diffusion electrode. The effects of the activation, mass transport and ohmic overpotentials were simulated via an "effectiveness factor" approach. The macro-homogeneous model was suitable to simulate the ORR kinetics at the RDE. On the other hand, it was found that the macro-homogeneous model does not simulate the operation of a porous gas diffusion cathode in PEMFC. With this model, the diffusion overpotential in the cathode is considerably overestimated. Conversely, the good agreement between calculated and experimental Tafel plots demonstrates the validity of the agglomerate model, even though the active layers of the PEMFC electrodes were thin and contained no PTFE. These results provided evidence for a two step transport process in the active layer of PEMFC electrodes.