A Proton Exchange Membrane (PEM) electrolyser is characterised and modelled to identify important electrochemical effects. These electrochemical effects include the ohmic, activation, and concentration losses within the PEM electrolyser during hydrogen production. The electrochemical effects of the PEM electrolyser are modelled by means of equivalent electrical circuits. The equivalent electrical circuit components of importance are the membrane resistance, the charge transfer resistance, the double layer capacitance and the Warburg impedance. The current interrupt method is proposed by using, (i) the natural voltage response method and (ii) system identification, for solving the parameters of the equivalent electrical circuits. In this paper both simulation and experimental results are provided.