The analysis of the electrochemical behaviour of polymer electrolyte fuel cells (PEFC) both in time and frequency domain requires appropriate impedance models. Simple impedance models with lumped parameters as resistances and capacitances or Warburg impedances do have limitations: often the validity is limited to a certain frequency range, effects at very low or very high frequencies can not be described properly. However, these models have their usefulness for engineering applications. e.g. to distinguish the major loss terms, to estimate the membrane resistance, and to observe the changes of internal losses of fuel cells over time without the need for additional sensors. The work discusses different impedance configurations and their applicability to impedance spectra of a fuel cell stack. Impedance spectra at points along the DC polarization curve, as well as spectra at Various operating conditions are analysed and identified by a complex nonlinear least squares method. Finally, the connection of the impedance data with and the assignment of the parameters to physical phenomena are discussed. The examination shows that simple impedance models are well qualified to describe the electrochemical behaviour over a wide frequency range at all operating conditions. (C) 2008 Elsevier B.V. All rights reserved.