In this work, the thin-film /flooded-agglomerate model of gas diffusion electrodes was used to study the oxygen reduction reaction (ORR) on rotating disk electrodes with thin porous coating in alkaline solutions. Vulcan XC-72 carbon and 10, 20, and 30% w/w Pt/C catalysts were used as electrode materials. Fitting of the theoretical equations to the experimental data was employed to simulate the polarization characteristics of the ORR on the catalysts and to obtain the oxygen concentration profiles along the thickness of the flooded agglomerate as a function of the electrode potential and the structural parameters of the electrodes. The method was found to be a simple and adequate way to evaluate the performance of the supported catalysts. The predicted effect of doubling of the Tafel slope of the ORR due to diffusion of the reactant in the flooded agglomerate was confirmed experimentally. The kinetic parameters obtained for the ORR in alkaline media indicated that, for the lower Pt/C ratios, there is a strong participation of the carbon substrate in the catalysis of the reaction.