Hydrogen production via high temperature steam electrolysis is a promising technology as it involves less electrical energy consumption compared to conventional low temperature water electrolysis, as consequence of the more favourable thermodynamic and electrochemical kinetic conditions for the reaction. This paper reports on the Solid Oxide Electrolyser Cell (SOEC) performance as function of the operating parameters temperature, humidity and current density. Current-voltage measurements are coupled with impedance spectroscopy, in order to identify the different loss terms in the cell behaviour coming from the electrolyte resistance and the electrode processes. Remarkably high electrical-to-hydrogen energy conversion efficiencies are achieved (e.g., cell voltages of 1.0 and 1.25 V at -1A cm(-2) and 900 and 800 degrees C, respectively). Results obtained, moreover, show that an important limitation for the electrolysis reaction, at least at moderate absolute humidity values below about 70 vol.% can be the steam diffusion in the hydrogen/steam electrode. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.