The paper presents the results of a voltammetric and rotating ring disk electrode (RRDE) study of oxygen reduction reaction (ORR) catalyzed by Fe and Co porphyrins and phthalocyanines adsorbed on a high surface area carbon (Vulcan XC72) and by carbon supported Pt catalysts in presence of various quantities of Nafion (R). The results demonstrate that the hydrophobic backbone of Nafion (R) self assembles on nanoparticles of common carbon supports of ORR catalysts. The phenomenon is promoted by the attractive interactions of the backbone with the graphitic surfaces of carbon particles. It leads to a significant ORR inhibition as a result of the spillover of the hydrophobic Nafion (R) component onto the catalyst particles. The extent of the inhibition depends on the type and the amount of the catalyst on the carbon surface and the degree of carbon surface graphitization. The activity of the transition metal macrocycles is suppressed by up to two orders of magnitude, whereas that of low Pt loading (4.8%) catalysts by less than one order of magnitude. The results demonstrate a great risk of incorrect catalyst activity determinations when using even very small Nafion (R) quantities as the catalyst ink dispersing agent (C) 2016 Elsevier B.V. All rights reserved.