The purpose of this paper is to report on the electrochemical characterization of Polymer Electrolyte Membrane (PEM) water electrolysis cells. Results were obtained using membrane-electrode assemblies containing unsupported IrO2 catalyst at anode for the oxygen evolution reaction (OER) and carbon supported platinum nano-particles at the cathode for the hydrogen evolution reaction (HER). Roughness factors of anodes and cathodes have been determined using an internal reference electrode. Individual cell voltage contributions have also been measured as a function of operating current density. Cell impedance spectra have been measured at different cell voltages along the polarization curve. It is shown that charge transfer processes are major cell impedance contributors at voltages up to 1.8-1.9 V. At higher cell voltages, cell impedances are mainly resistive. It is shown that the impedance associated with the HER is negligible and that the two time-constants observed on experimental impedance spectra can both be attributed to the OER. Possible mechanism options are discussed. Finally, some results related to the EIS characterization of PEM water electrolysis stacks are also reported. (C) 2014 Elsevier Ltd. All rights reserved.