The mass transfer properties of Nafion(R) coated Pt rotating disk electrodes (rde) towards ferricyanide and ferric ions were investigated. The influence of the film thickness, of the reactant concentration and of the rotation speed were qualitatively well described by the membrane model. This model, however, did not perfectly fit the experimental results; especially, the increase of the rde rotation speed corresponded to a higher increase of the limiting current than predicted by the model. This deviation was particularly important at high rotation speeds. The presence of inhomogeneities or pinholes is proposed to explain this behaviour.