The effect of oxygen depletion underneath the ribs of a polymer electrolyte fuel cell (PEFC) air cathode on the impedance response is evaluated by using locally resolved electrochemical impedance spectroscopy (EIS) in channel and land areas of a differential H-2/air PEFC with submillimeter resolution as well as two-dimensional impedance modeling. At a higher cell polarization, we observed a strong increase of the integral cell impedance toward higher cell current. Our results demonstrate that this phenomenon is primarily a consequence of oxygen depletion underneath the ribs, while the contribution of the channel region is of minor importance. This underlines that the use of small active area PEFCs in EIS measurements does not generally justify a one-dimensional treatment of data by using through the membrane electrode assembly impedance models, even if these cells are operated at high gas flow rates. In differential cells, only the channel center can be regarded as a quasi one-dimensional cell. The contribution of in plane processes to PEFC impedance response must be seriously taken into account. Simply focusing on through plane processes runs the risk of considering the wrong phenomena and can lead to wrong conclusions. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3536498] All rights reserved.