Our experimental results shown here disprove that finite diffusion can generally be assumed in ac impedance models for H-2/air-polymer electrolyte fuel cells (PEFCs) to account for the diffusive transport of oxygen through the gas diffusion layer (GDL) toward the air electrode. It is shown that the amplitude of the oxygen concentration oscillation created as a consequence of superimposed ac current at the air electrode is not zero at the channel/GDL interface but extends into the gas channels, at least below modulation frequencies of f(mod)=10 Hz. By this, sinusoidal oxygen-concentration oscillations within the cathode gas channels are excited locally along the flow field. Due to the forced air convection in the cathode flow-field channels, a coupling via the gas phase occurs downstream of the flow field. The coupling strongly affects the local and by this the overall impedance response of the cell and evokes the formation of a low-frequency arc in H-2/air-PEFC impedance spectra. Based on the experimental results, a qualitative model is presented explaining the local impedance response of a segmented 200 cm(2) H-2/air PEFC.