Mechanistic studies of oxygen reduction reaction (ORR) on electrocatalysts derived from the pyrolysis of iron salts and polyethyleneimine (PEI) were performed in acidic and alkaline media. The prepared materials have been characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area analysis, and rotating ring-disk electrode (RRDE) methods. Systematic studies of catalyst loading on working electrode allowed establishing a working range suitable for mechanistic studies. It was found that the overall mechanism of ORR in acid electrolyte is not identical with that in alkaline media indicating complex interplay. The dependence of electron transfer numbers on catalyst loading indicates 2 x 2e(-) mechanism. The analysis of the Tafel plots corresponding to electrode polarizations have demonstrated difference of the reaction rate limiting steps in alkaline and acid media. (C) 2013 Elsevier B.V. All rights reserved.