Electrochemical in situ X-ray diffraction measurements were performed in 1 N H2SO4 electrolyte on a thermally prepared RuO2 thin layer deposited on a Ti substrate. As evidenced by a shift of the characteristic diffraction peaks of RuO2, a volume increase of the tetragonal unit cell occurs when the electrode is polarized in the potential region where hydrogen evolution occurs. These changes in the lattice parameter are thought to occur as a result of hydrogen absorption in the oxide layer. These modifications are not totally reversible, as the positions of the diffraction peaks of the used electrode do not coincide with that of pristine material. (C) 2002 The Electrochemical Society.