This paper presents a study of the oxygen reduction reaction on platinum on carbon thin porous coating rotating disk electrodes in 1.0 M NaOH and 0.5 M H2SO4 solutions using ac impedance spectroscopy. The results are analyzed and simulated with the thin film/flooded agglomerate model. It is observed that the experimental system presents only one semicircle on Nyquist plots, because the processes in the agglomerate and the diffusion in the thin film are masked by the large value of the double layer capacitance. However, fitting of the impedance plots with the theoretical model leads to the conclusion that the processes in the agglomerate and in the thin film play important roles on the behavior of the electrode. The kinetic parameters obtained from the ac measurements are totally consistent with those observed in the de experiments.