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Abstract: This paper describes the stability of Fe species in a heat-treated polyaniline–iron–carbon (PANI–Fe–C) oxygen reduction reaction (ORR) catalyst in an aqueous acidic electrolyte and in a membrane–electrode assembly (MEA) at various potentials. Linear combination fitting of ex situ and in situ X-ray absorption near-edge structure (XANES) spectra to the spectra for a suite of Fe standards was used to determine the catalyst iron speciation at various potentials, after potential cycling in an aqueous electrolyte, and after 200h potentiostatic holds in MEAs. XANES edge-step analysis and inductively-coupled mass spectrometry were used to determine the amount of Fe lost from the catalyst into the aqueous electrolyte and from the MEA cathodes. Results showed that the Fe was lost from the catalyst in the electrochemical environment and the rate and extent of this loss were dependent on potential and on the type of electrolyte. The Fe specie primarily responsible for this loss was iron sulfide. Despite the large overall loss of Fe species from the catalyst that had been subjected to potentiostatic holds in an MEA at either 0.4V or 0.6V for 200h, H2–air polarization curves showed only moderate loss of cathode kinetic performance while the performance in the mass transport region improved. Correlating the performance loss to the XANES speciation, the kinetic losses may be attributed to the oxidation of active site(s) and/or loss of pyrrolic-like and pyridinic-like coordination, as well as the mass transport improvement due to removal of inactive Fe species, predominantly sulfides. Species with porphyrazin-like coordination were stable in both the aqueous and MEA environments. It is speculated that the stability of the porphyrazin is responsible for the durability of this catalyst. [Copyright &y& Elsevier]