The effect of oxygen depletion along the cathode flow channels on the local impedance response of a polymer electrolyte fuel cell (PEFC) operated on air and pure hydrogen was investigated using a novel experimental approach. The method combines the use of sectioned electrodes and local ac measurements in PEFCs. The results shown here give experimental proof that oxygen concentration oscillations in the cathode gas channels generated as a consequence of ac current flow evoke the formation of a low frequency capacitive loop in local H-2/air-PEFC impedance spectra in downstream parts of the flow field. The loop shows positive or negative real parts at low frequencies depending on the air stoichiometry. The loop is not observed in the local spectrum when the ac current is applied locally and, consequently, the impact of upstream processes on the local impedance response is excluded. A mechanistic description of the processes occurring during ac measurements in H-2/air-PEFCs was developed and validated by experiments. The results show that processes occurring along the flow channels of a PEFC cannot be excluded from the interpretation of PEFC impedance spectra even at rather high air stoichiometries and are in fact the most important phenomena at low modulation frequencies (f(mod)< 10 Hz). (c) 2007 The Electrochemical Society.