A method is proposed to model the electrochemical characteristics of a Proton Exchange Membrane (PEM) electrolyser. The electrochemical characteristics, which include the Ohmic, activation and concentration losses, are modelled by means of an equivalent electric circuit impedance. The equivalent electric circuit impedance under consideration is the Randles Warburg (RW) cell and the parameters are obtained through the current interrupt (CI) method. The CI method consists of two parts, 1) the natural voltage response (NVR) method to model the Ohmic losses, and 2) the current switching (CS) method to model the activation and concentration losses. A simulation model of the RW cell is used to verify and validate the CI method. Thereafter, the CI method is practically implemented and the results presented. Results show that the Ohmic losses correlates well with existing literature. The paper furthermore illustrates the capability of the CI method to visually illustrate the RW impedance through Nyquist diagrams. Nyquist diagrams are used to illustrate the concentration losses and indicate the degradation state of the PEM at specific operating conditions. Results show that the concentration losses decrease with an increase in the operating current density and temperature. Copyright (c) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.