The surface composition of a thermally prepared RuO2 electrode was studied by X-ray photoelectron spectroscopy (XPS) to investigate structural and surface composition changes occurring during the hydrogen evolution reaction. This was done using an electrochemical cell (EC) attached directly to the ultra high vacuum (UHV) chamber of the spectrometer, allowing the direct transfer of the electrode from the EC to the UHV chamber without exposure to ambient conditions. All the treatments have been performed in this cell, using IN H2SO4. After a polarization of the electrode to -0.5 V versus standard calomel electrode (SCE), the XPS spectrum showed no shift in the binding energy of the Ru core level peaks, indicating that no reduction of Ru(IV) occurs. Further analysis of the O Is core level spectrum also revealed that the adsorption of sulfate anions is maximum at -0.5 V versus SCE. (C) 2003 Elsevier Ltd. All rights reserved.