This work discusses the electrocatalysis of the oxygen reduction reaction on platinum on carbon thin porous coating rotating disk electrodes in alkaline and acid media. The electrochemical techniques considered are cyclic voltammetry, steady state polarization and impedance spectroscopy. Both the de and ac polarization results are analyzed and simulated with the thin film/flooded agglomerate model. Cyclic voltammetry allows an inspection of the crystal facet structure of the Pt particles and the determination;;of effective surface areas which are lower than those determined from X-ray diffraction techniques or transmission electron microscopy. The de polarization and impedance results show clearly a duplication of the Tafel slope due to structural effects on the porous electrode. Experiments in alkaline solutions show that in this medium there is an important contribution from the carbon to the kinetics of the reaction, which is negligible in acid medium. The effect of Pt particle size on the oxygen reduction electrocatalysis in both electrolytes is correlated with the predominant facets of the platinum crystallites.