This work aims at analysing the performances of a prototype of a high pressure Polymer Electrolyte Membrane water electrolyser. The whole work was funded in the framework of the Italian FISR (Research special supplementary funding) project. The high pressure electrolyser prototype was manufactured by Giner Inc. (Massachusetts, USA), whereas the test bench was expressly designed and built thanks to the collaboration with LAQ INTESE (High Quality Laboratory - Technological Innovation for Energetic Sustainability) and with Sapio SpA (Monza, Milan, Italy). An electrochemical model of the electrolyser stack is developed, which calculates the theoretical open-circuit voltage via a thermodynamic analysis of the process, and then obtains the expected real voltage during operation by calculating the different overvoltages as function of the current. The final result of the model is a theoretical polarisation curve. Experimental results are then presented and compared with the model theoretical results, and thanks to an experimental data fitting it is possible to obtain the estimated values of some important process parameters and their trend in different temperature and pressure conditions. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.