This paper investigates a single complete proton exchange membrane fuel cell (PEMFC) as a whole, modelling it by an electrical equivalent circuit (EC) using the electrochemical impedance spectroscopy (EIS) technique. The cell tested was a PEM single cell with a Nation 117 membrane. The measurements were carried out with the fuel cell working in an electrode "symmetrical mode" (SM), that is, with the same gas injected in both electrodes, with different gases supplied to anode/cathode compartments (O-2/O-2 and H-2/H-2). The working temperature of the cell was (40-70 degrees C), and the humidification conditions of reactant gases were 45 +/- 5 and 100% relative humidity (RH). This study obtains the structure of the EC and its parameters values, and offers a physical interpretation relating these parameters to several processes occurring in the system. The relations of properties, such as membrane conductivity or the resistance to the charge transfer process as a function of the working parameters of the fuel cell, such as temperature, reactant gases injected and their humidification conditions are also studied. The efficiency of EIS for the modelling and characterisation of PEMFC working in SM is also discussed. (c) 2005 Elsevier B.V. All rights reserved.