In this study, an impedance model based on electrochemical theory considering hydrogen peroxide formation during a two-step oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) has been developed. To validate the theoretical treatment, electrochemical impedance spectroscopy (EIS) measurements were carried out in an open-cathode 16 cm(2) H-2/air PEFC stack. The results show that inductive loops at low frequencies of the impedance spectra are attributed to mechanisms related to hydrogen peroxide formation during ORR. The results also demonstrate that the mechanisms during consumption of hydrogen peroxide to form water (second-step in ORR) can be the dominating process for losses in the PEFC compared to the mechanisms during oxygen consumption to form hydrogen peroxide (first-step in ORR). Oxygen transport limitations can be a result of hydrogen peroxide adsorbed onto the surface of the electrode which reduces the number of active sites in the cathode catalyst layer for oxygen to react. This study could support results from other experimental techniques to identify hydrogen peroxide formation during the ORR that limit the performance of PEFCs. (C) 2015 Elsevier B.V. All rights reserved.