One of the most demanding research topics related to the Polymer Electrolyte Membrane Fuel Cell (PEMFC) concerns its reliability. Apart from the security aspects, it is basic to have a diagnosis of the internal state of the PEMFC in order to correct and optimise its operation. The Fuel cell state and response depends on the imposed operating conditions, which are mainly given by temperatures, pressures, humidity, reactants concentrations and current. This work explores the use of fuel cell experimental Electrochemical Impedance Spectroscopy (EIS) as a tool to characterise the fuel cell state, what can be very helpful for diagnosis purposes. With this objective in mind, a definition of “relevant characteristics” extracted from EIS response is done. These “relevant characteristics” can be used in order to characterize the fuel cell and also to find the parameters of simple equivalent circuits of its dynamical response. Besides, a complete equivalent circuit which permits a close fitting of the EIS response for all operating conditions is proposed and its evolution with operating pressure is studied.